gfapi: wake migration waiters after migration

Wake call threads which are waiting on migration to complete with
a cond_broadcast. Else if any call which arrives right when migration
is attempted will end up hanging indefinitely.

Change-Id: I7df5298f93998d9a54fb12c16654e62333018ece
BUG: 953694
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/6111
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-by: Shyamsundar Ranganathan <srangana@redhat.com>