api: ConnectionPool.Add()/ConnectionPool.Remove()

* ConnectionPool.Add() allows to add a new endpoint into a pool.
* ConnectionPool.Remove() allows to remove an endpoint from a pool.

Closes #290

Author: oleg-jukovec

CHANGELOG.md

@@ -14,6 +14,8 @@ Versioning](http://semver.org/spec/v2.0.0.html) except to the first release.
   requests to complete (#257)
 - ConnectionPool.CloseGraceful() unlike ConnectionPool.Close() waits for all
   requests to complete (#257)
+- ConnectionPool.Add()/ConnectionPool.Remove() to add/remove endpoints
+  from a pool (#290)
 
 ### Changed