* 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.

Add more words about how to specify the zone:port in a Corefile.

Fixes #489

* Add pod cache and verified pod responses

* add ip indexing for pod cache

This blows up prometheus (panic in CoreDNS) and is not that
handy to have.

Add txt response per k8s spec

By defining and using an proxy.Exchanger interface we make the proxy
more generic and we can then fold back httproxy into proxy.

This overrides #463 and #473 and should make futures extensions rather
trivial

* Add docs that talk about `protocol` and how to set it.
* middleware/proxy: rename New to NewLookup
  It's used as a Lookup mechanism not as a completely new proxy,
  reflect that in the name.
* Set maxfails to 3 by default when looking up names.

Most of the changes have been copied
from https://github.com/johnbelamaric/coredns/pull/1/files

* dont require/allow "_" prefix for srv wildcard fields

* streamline parse/validation of req name

* removing nametemplate

* error when zone not found, loopify unit tests

* return servfail for pod rqsts when pods disabled

* Add integration test for disabled pod mode

* Use common TLS parsing routine for etcd

Change to use the new common routine, and update the docs to reflect
the different options for passing TLS configuration.

* Move middleware/tls to middleware/pkg/tls

This was put in the wrong place originally.

Create a small speedup running the tests:

PASS
ok  	github.com/miekg/coredns/test	10.329s

PASS
ok  	github.com/miekg/coredns/test	6.079s

Skip the etcd ones. Doing the middleware/*/*_test ones doesn't yield
any speedup as these are still done on a per directory basis.

Add deadline to break the connection. We use the default of 5 seconds.
After this the backend is marked unhealthy and not used for some time.

Fixes #467