Check schema version (stored in box.space._schema) on start and
print a warning if it doesn't match last available schema version.
It is needed because some users forget to call
box.schema.upgrade() after Tarantool update and get stuck with an
old schema version until they encounter some hard to debug
problems.

Closes #4574

Co-developed-by: Roman Khabibov <roman.habibov@tarantool.org>