wal: fix wal_queue_max_size assignment during initial box.cfg

wal_queue_max_size took effect only after the initial box.cfg call,
meaning that users with non-zero `replication_sync_timeout` still synced
using the default 16 Mb queue size. In some cases the default was too
big and the same issues described in #5536 arose.

Fix this.

Closes #10013

NO_DOC=bugfix

(cherry picked from commit ab0f7913)