recovery: refactoring, move snap_io_rate_limit to memtx_engine

* do not use snap_dir to load bootstrap.snap, just any dir object
  will do
* move snap_io_rate_limit to memtx_engine
* remove box_set_panic_on_wal_error(), the dynamic
  setting is used for relays, there is no point
  to update the global recovery instance after box.cfg{}