fix: use tarantool's Mutex instead of RefCells

Previously sbroad used ordinary Rust's `RefCell` for smart pointer behaviour.
It's pretty dangerous in tarantool environment, as it will be human responsibility to
check that no yield happens before `RefCell` is unlocked.

It causes problems with, for instance, cartridge's `apply_config` during live load - 
it needs to exclusively borrow the metadata, but it's already borrowed by ordinary insert requests.