Commit 14e8803f removed the locking in SyncRepWaitForLSN, but that
introduced a race condition, where SyncRepWaitForLSN might see
syncRepState already set to SYNC_REP_WAIT_COMPLETE, but the process was
not yet removed from the queue. That tripped the assertion, that the
process should no longer be in the uqeue. Reorder the operations in
SyncRepWakeQueue to remove the process from the queue first, and update
syncRepState only after that, and add a memory barrier in between to make
sure the operations are made visible to other processes in that order.

Fixes bug #14721 reported by Const Zhang. Analysis and fix by Thomas Munro.
Backpatch down to 9.5, where the locking was removed.

Discussion: https://www.postgresql.org/message-id/20170629023623.1480.26508%40wrigleys.postgresql.org