When the last indexing operation is completed, we will fire a global
checkpoint sync.  Since a global checkpoint sync request is a
replication request, it will acquire an index shard permit on the
primary when executing.  If this happens at the same time while we are
issuing the synced-flush, the synced-flush request will fail as it
thinks there are in-flight operations. We can avoid such situation by
not issue the synced-flush until the global checkpoint on the primary is
propagated to replicas.