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
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
* middleware/proxy
Use connection pooling for communicating with an upstream, instead of
opening a new socket every time.
This makes the proxy more efficient and allowed for some cleanups.
* Some cleanups
* Some fixes
* more
* Kill pool
* Add nil check
* remove pool
