box: fix schema downgrade replication

Some downgrade operations are performed with disabled system space
triggers because they were prohibited recently (creation of SQL built-in
functions) or never allowed (dropping a system space). This works fine
on the instance running downgrade but apparently fails on replicas.

To fix this issue, let's disable the checks the operations that prevent
downgrade in the following scenarios:
 - in the fiber that is currently running a schema upgrade or downgrade;
 - in the applier fiber so that it can replicate changes done by upgrade
   or downgrade on the master;
 - during recovery so that DDL records written to the WAL can be
   replayed.

We already have all the necessary infrastructure in-place - we use it
for allowing DDL operations with an old schema for upgrade.

Closes #9049

NO_DOC=bug fix