api: add ConnectionHandler to the connection_pool

ConnectionHandler provides callbacks for components interested
in handling changes of connections in a ConnectionPool.

We have to take into account that user callbacks can take an
indefinite amount of time. The simplest solution is to process each
connection in a separate goroutine. This should not affect to
performance because most of the time these goroutines are blocked.

Closes #178

Author: oleg-jukovec

CHANGELOG.md

@@ -11,6 +11,8 @@ Versioning](http://semver.org/spec/v2.0.0.html) except to the first release.
 ### Added
 
 - Support queue 1.2.0 (#177)
+- ConnectionHandler interface for handling changes of connections in
+  ConnectionPool (#178)
 
 ### Changed