box: fix failing assertion in box_promote_qsync

Fixed a bug when the assertion in box_promote_qsync would fail.
The assertion is that at the moment when box_promote_qsync is
executed, no other promote is executed. It turned out that this
assertion is basically incorrect. Now after this patch the newly
elected leader is trying to repeat box_promote_qsync in
box_raft_update_synchro_queue until it fails due to the fact
that some other promotion is currently being executed.

Closes #9263

NO_DOC=bugfix