fix: use tarantool's Mutex instead of RefCells

Summarize the changes

• fix: use tarantool's Mutex instead of RefCells

Previously, RefCells were used for protecting shared data. It is dangerous in the tarantool environment, hence we move to Mutex.

It fix, namely, problem with apply_config calls during cluster load.

Ensure that

- [ ] New code is covered by unit and integration tests. - [ ] Related issues would be automatically closed with gitlab's closing pattern (Closes #1, #2). - [ ] Public modules are documented (check the rendered version with cargo doc --open). - [ ] (if PEST grammar is changed) EBNF grammar reflects these changes (check the result with railroad diagram generator.

Next steps

• Update sbroad submodule in picodata/picodata.
• (if EBNF grammar is changed) create a follow-up issue in picodata/docs.