BUG-3516: make PingPongTransactionChain.close() asynchronous

BUG-3516: make PingPongTransactionChain.close() asynchronous

When the system is critical loads Thread.yield can bring a long-term
blocking effect for hijacked threads like Netty. We should not be
blocking for prolonged time.

Rework the shutdown logic to be asynchronous, and scheduling the
potential outstanding transaction to complete as appropriate. Also fixes
the case where we would end up not reporting a transaction failure if
the transaction is readied, but was not submitted to the backend.

Change-Id: Ic7796a980d9e87242f70b7f7b9cdb30caeab9dd9
Signed-off-by: Vaclav Demcak <vdemcak@cisco.com>
Signed-off-by: Robert Varga <rovarga@cisco.com>