Health overloaded metrics does not carry the server label. Explain why.

* global: move to context

Move from golang.org/x/net/context to std lib's context.

Change done with:

for i in $(grep -l '/context' **/*.go); do sed -e 's|golang.org/x/net/context|context|' -i $i; echo $i; done
for i in **/*.go; do goimports -w $i; done

* drop from dns.pb.go as well

Remove trailing white space from proxy/README.md

* adding sequential policy to forward plugin

* making sequential an alias to first in proxy plugin

* Update all plugins to use plugin/pkg/log

I wish this could have been done with sed. Alas manually changed all
callers to use the new plugin/pkg/log package.

* Error -> Info

* Add docs to debug plugin as well

* pkg/log: ability for debug logs

When the debug plugin is enabled all log.Debug calls will print to
standard; if not there are a noop (almost).

The log package wraps some standard log functions as well, so just
replacing "log" with "plugin/pkg/log" should be enough to use this
package.

* docs

* Add docs

* lint

* Test fallthrough to log pkg as well

* simple package - up test coverage

* add other log levels as well

* update docs

* check for bare zone query

* check for bare type query

* plugin/metrics: add 'server' label

This uses the new WithServer(ctx) to get the current server from the
context.

First in a larger refactor to make all plugins do this.

* compile

* compile

* lala test

* compile and test

* typos

* Dont duplicate the code

* Move to readtimeout

* lets compile

* address comment

* comment from pr

* much smaller minimum

if GOPATH is not set use $HOME/go which is the current default in Go.

Fixes #1642

Stop all goroutines after we get the stop signal.

With this change the original truncated message returned by requested
server is returned to the client, instead of returning an empty dummy
message with only the truncation bit set.

* plugin/forward: check TC correctly on reply.

Add test for this.

* Add proxy test as well

This allows to log responses of different classes, for example, denial and error.

- each proxy stores average RTT (round trip time) of last rttCount queries.
   For now, I assigned the value 4 to rttCount
 - the read timeout is calculated as doubled average RTT, but it cannot
   exceed default timeout
 - initial avg RTT is set to a half of default timeout, so initial timeout
   is equal to default timeout
 - the RTT for failed read is considered equal to default timeout, so any
   failed read will lead to increasing average RTT (up to default timeout)
 - dynamic timeouts will let us react faster on lost UDP packets
 - in future, we may develop a low-latency forward policy based on
   collected RTT values of proxies

* - adding policy first in the policies description

* - fix number of policies available

* plugin/forward: retry on cached tcp connection closed by peer

* fix linter warnings

* fixed unit test

* modify error message

This double checking for 1.9 and 1.10 is nice, but doubles the
travis test duration.

* plugin/forward: TCP conns can be closed

Only when we read and get a io.EOF we know the conn is closed (for TCP).
If this is the case Dial (again) and retry. Note that this new
connection can also be closed by the upstream, we may want to add a
DialForceNew or something to get a new TCP connection..

Simular to #1624, *but* this is by (TCP) design. We also don't have to
wait for a timeout which makes it easier to reason about.

* Move to forward.go

* doesnt need changing

Tagged an new release; has some fixes.

Return the client error if there was one instead of the generic
"no healthy upstream or error"

This registers the Collectors iff the metrics plugin has been loaded.
Safes a bunch of code in each and every plugin's setup code.

* plugin/metrics: set server address in context

Allow cross server block metrics to co-exist; for this we should label
each metric with the server label. Put this information in the context
and provide a helper function to get it out.

Abstracting with entirely away with difficult as the release client_go
(0.8.0) doesn't have the CurryWith functions yet. So current use is like
so:

define metric, with server label:

	RcodeCount = prometheus.NewCounterVec(prometheus.CounterOpts{
		Namespace: plugin.Namespace,
		Subsystem: "forward",
		Name:      "response_rcode_count_total",
		Help:      "Counter of requests made per upstream.",
	}, []string{"server", "rcode", "to"})

And report ith with the helper function metrics.WithServer:

	RcodeCount.WithLabelValues(metrics.WithServer(ctx), rc, p.addr).Add(1)

Drop the doTrue and doFalse and use a pointer to a bool to do a proper
tri-bool.

Check for trunacted in the lookup function as well and use the Match
function here as well.

My routers acts funny when it sees it non RD query; make this HC as
boring as possible

* plugin/{cache,forward,proxy}: don't allow responses that are bogus

Responses that are not matching what we've been querying for should be
dropped. They are converted into FormErrs by forward and proxy; as a 2nd
backstop cache will also not cache these.

* plug

* add explicit test

* request.Scrub: test for rl==size case

Make a test case for the new break statement in Scrub and also
account for the OPT record that may get re-added in SizeAndDo() -
otherwise we may break clients that expect this.

* Fix comment

Signed-off-by: Mario Kleinsasser <mario.kleinsasser@gmail.com>

Underscore these names.