box: fix an assertion failure in box.ctl.promote()

box.ctl.promote() used to assume that the last synchronous entry is
already written to WAL by the time it's called. This is not the case
when promote is executed on the limbo owner. The last synchronous entry
might still be en route to WAL.

In order to fix the issue, wait until all the limbo entries are written
to disk via wal_sync(). After this happens, it's safe to proceed to
gathering quorum in promote.

Closes #6032