Clarify which ID is used by DB driver's find_issue_by_id method
Clarify which ID is used by DB driver's find_issue_by_id method

It uses MongoDB's ObjectId, not the issue ID concept. Thus,
argument is renamed from `id` to `_id` to mark the difference and
align it with the rest of the code.

Fallout: Simplify ObjectId coversion code paths by dropping
unnecessary local variables (also affects remove_issue_by_id
method).

renatahodovan/fuzzinator,renatahodovan/fuzzinator,akosthekiss/fuzzinator,renatahodovan/fuzzinator,akosthekiss/fuzzinator,akosthekiss/fuzzinator,akosthekiss/fuzzinator,renatahodovan/fuzzinator