BUG-5280: expose queue messages during reconnect

BUG-5280: expose queue messages during reconnect

This patch reworks the internals of AbstractClientConnection
to isolate the TransmitQueue from the rest of the logic,
so we have proper split between implementation and interface
exposed to the users.

Furthermore the public interface is slightly reworked so the
individual Proxies have access to the (locked) queue contents,
which is needed to correctly replay transaction state within
transaction chains.

Change-Id: I1c08fa06eec4dd581e07002059c5142e6b0c1ed4
Signed-off-by: Robert Varga <rovarga@cisco.com>