trx_t::set_skip_lock_inheritance() must be invoked at the very beginning
of lock_release_on_prepare(). Currently  trx_t::set_skip_lock_inheritance()
is invoked at the end of lock_release_on_prepare() when lock_sys and trx
are released, and there can be a case when locks on prepare are released,
but "not inherit gap locks" bit has not yet been set, and page split
inherits lock to supremum.

Also reset supremum bit and rebuild waiting queue when XA is prepared.

Reviewed by: Marko Mäkelä