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When the origin wants to do a FLUSH sync, if there are
active-message operations that are going to be issued,
we piggback the FLUSH message with the last operation;
if no such operations, we just send a single FLUSH packet.

If the last operation is a write op (PUT, ACC) or only
a single FLUSH packet is sent, after target recieves it,
target will send back a single FLUSH_ACK packet;
if the last operation contains a read action (GET, GACC, FOP,
CAS), after target receiveds it, target will piggback a
FLUSH_ACK flag with the response packet.

After origin receives the FLUSH_ACK packet or response packet
with FLUSH_ACK flag, it will decrement the counter which
indicates number of outgoing sync messages (FLUSH / UNLOCK).
When that counter reaches zero, origin can know that remote
completion is achieved.
Signed-off-by: Pavan Balaji <balaji@anl.gov>