THRIFT-923. cpp: Add completion notification to async clients

THRIFT-923. cpp: Add completion notification to async clients

Add a virtual function "completed__(bool)" to xxxCobClient that is
called by recv_xxx() after reception of a response (arg = true) or an
exception (arg = false). This allows the TAsyncClient to intercede at
that point, permitting, e.g., the load-balancing of persistent
connections that would otherwise remain bound to a single server.

A new "no_client_completion" flag inhibits generation of this mechanism.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@1005131 13f79535-47bb-0310-9956-ffa450edef68