* linter fixes

* Golint and format code

* fmt

This fix fixes import path from
`github.com/miekg/coredns`
->
`github.com/coredns/coredns`

This fix fixes badge in README.md so that it will correctly reflect
github.com/coredns/coredns

We now sit under coredns/coredns this means the logo can be in it's
repository.

* middleware/metrics: survive restart

Keep the handler running during restart. Stopping and starting the
handler results in "address in use" - sometimes, meaning the reload
will be flaky. In turn this behavior means any changes to the monitor
stanza are not picked up.

* remove resync

* Document fallthrough and fix *reverse*

While documenting the fallthrough behavior and testing it I noticed
the did not properly work. This PR does a tiny bit too much as it

- Documents fallthrough
- Fixes fallthrough in reverse
- Makes directives_generate complain on duplicate priorities
- Moved reverse *before* file in middleware.cfg
- Add a test that tests the reverse fallthrough behavior with a file
  backend

Fixes #515

* ....and fix the tests

Add additional section processing for MX and SRV records. Update the
tests.

* all-middleware-equal

* Revert "all-middleware-equal"

This reverts commit ee77b2a9816b1953a19fefb863875399aacd0c2a.

* middleware: treat external and local the same

Make the middleware generation simpler and also specify the local
middleware, meaning that it can now be removed as well. Simplify
the code a bit and regen everything.

* remove lineNR, not used

* server: fix data race

This fixes the detected race.

Fixes #534

* Remove the listener and packetconn from Server

There does not seem a need to store the listener and packetconn again
in the Server structure. The dns.Servers already has access to them
and can also shutdown the handlers.

When using an external middleware, the generated file was failing to compile.
The issue is a typo in directives_generate.go that left imports with an
unterminated string.

* WIP: Client-side of gRPC proxy

* Add tests

* gofmt

* Implement OnShutdown; add a little logging

* Update for context in Exchange change

* go fmt

* Update README

* Review comments

* Compiling is good

* More README improvements

Remove the "gen" directory and move directives_generate.go out of it.
Add a build ignore tag so it isn't build by default. Cleanup the go gen
invocations so there are not seen as package docs.

Simplify the code a bit and don't run go gen twice.

* middleware/proxy: give Exchange a context

Make context.Context the first paramater in the Exchange method.
This is inline with all other query functions.

* up the version

* core: make coredns.Server a caddy.GracefulServer

We needed to also implement the Address() method.

Fixes: #519

* Add compile time check if we implement caddy.GracefulServer

* Check if we should shutdown

* middleware/reverse: random updates

Make the documentation somewhat shorter (and hopefully clearer in the
process). Also to be on-par with the *auto* middleware, start counting
the referenced zones from 1 (instead of 0).
Some variable cleanups and use the NextOrFailure in the ServeDNS
function.

* More TODOs

* fix k8s next middleware chaining

* add chain to integration tests

* if nit

* Require Field for rewrite rules

* review feedback changes

* fix ut

* fix typo, add warning message

Add proxy_proto and re-instate proto to be the protocol of the incoming
query ("tcp" or "udp").

* check for no namespace filter

* integration test

* set zone for cidr based PTRs

* set zone to request

Fix the except keyword usage - the config would allow it, but it was
not enforced in the code.
Turns out that **FROM** was also not enforced, fix both, by (basically)
copying the code from Caddy.

Update the README and tests.

Locally test as well, shows that this works:

~~~
.:1053 {
    proxy miek.nl 8.8.8.8:53 {
        except a.miek.nl
    }
    proxy a.miek.nl 8.8.4.4:53

    errors stdout
    log stdout
}
~~~

And gives the desired results, not having a proxy line for `a.miek.nl`
results in a SERVFAIL (as expected).

Fixes #502

* middleware/proxy: absorb httpproxy

Move the httproxy into proxy. This adds and Exchanger interface which
is used to exchange the messages with the upstream.

The https_google upstream will re-resolve itself and update the upstream
hosts used every 300s.

* Remove and add TODO

* add cidrs opt

* remove state data from middleware object

* update k8s docs

* Add integration tests

* add unit tests for cidr and pods config

* more README fixes, separate dev notes

* adjust section headers

* fix typo

* add cidrs opt

* remove state data from middleware object

The gRPC server middleware[1] needs access to the Server object
in order to push the unpacked Msg through the normal middleware
pipeline. These are the changes to core needed to make that
possible.

[1] https://github.com/infobloxopen/coredns-grpc

* Use go generate to build middleware setup based on middleware.cfg
Init default config

* generated files

* Move gen to an isolated area

* rename files

* PR review updates

* undo readme

This fix fixes several `go fmt`, `go lint`, and `go vet` issues,
to make goreportcard happy:

https://goreportcard.com/report/github.com/miekg/corednsSigned-off-by: Yong Tang <yong.tang.github@outlook.com>

Adds a middleware to enable tracing with OpenTracing/OpenZipkin.
Enabling tracing will have a large impact on performance so it is
not advisable in production.