BUG-5280: Create AbstractProxyHistory class

BUG-5280: Create AbstractProxyHistory class

Given the connection-oriented nature of SequencedQueue, we
really need to properly encapsulate various aspects of the client,
so we can perform proper state propagation, both during message
transmission and on reconnection.

This is a first step in that direction, which encapsulates client's
sendRequest() and self() methods at proper levels. It furthermore
makes state tracking in proxies consistent with state tracking in
their aggregate counterparts, hence each ProxyTransaction is guaranteed
to have an associated ProxyHistory.

Change-Id: I8c15b234ec813ac427e63a6e077ae17cde443be3
Signed-off-by: Robert Varga <rovarga@cisco.com>