raft: finish worker fiber on Tarantool shutdown

Let's make sure box raft worker fiber is finished on Tarantool shutdown as
we are going to free fibers stacks. If fiber is not finished it's stack
may have references to objects on heap. Thus as fiber stack will be freed
we will have FP memory leaks under ASAN.

Part of #9722

NO_TEST=rely on existing tests
NO_CHANGELOG=internal
NO_DOC=internal