Base prevention of replay attacks on proxy

DualStack is deprecated

Add Golang 1.12 to TravisCI

```
	// DualStack previously enabled RFC 6555 Fast Fallback
	// support, also known as "Happy Eyeballs", in which IPv4 is
	// tried soon if IPv6 appears to be misconfigured and
	// hanging.
	//
	// Deprecated: Fast Fallback is enabled by default. To
	// disable, set FallbackDelay to a negative value.
	DualStack bool
```

Update README.md

- little typo in docker command line 
- added more preferred 'secure only' example

Remove generating of version.go

Add explanation on secure mode

tiny fix README.md

Hopefully, this closes https://github.com/9seconds/mtg/issues/49

Use mtg metrics only for prometheus endpoint

Log configuration on proxy start

This has to simplify the debugging because usually we do not know all
options which user used to start mtg.

Prometheus integration

Rework stats server

Decrease the number of used goroutines