box: ensure fiber processing box_cfg doesn't process messages from iproto

In box_cfg() we have a call to gc_set_wal_watcher(), which creates pipes
between 'wal' and 'tx' under the hood using cbus_pair().
While pipes are being created, the fiber calling gc_set_wal_watcher()
will process all the messages coming to 'tx' thread from iproto. This is
wrong, since we have a separate fiber pool to handle iproto messages,
and background fibers shouldn't participate in these messages
processing. For example, this causes occasional credential corruption in
the fiber executing box_cfg().

Since tx fiber pool is already created at the time gc_set_wal_watcher()
is called, we may forbid message processing for the fiber which calls
the function, one of the tx fiber pool fibers will wake us up when the
pipes are created.

Closes #3779