[FR] Override global log level for a functional area

[Chardrazle]

We had a scenario where the log produced from the SRT_LOGFA_BUF_RECV functional area produced a lot of noise ("RCV-DROPPED") at warning level.

We have several options to reduce this noise; however, none of them are ideal. For example:

• Adjust the global logging level.
  This has the danger of then missing information across the board.
• Switch off logging for the functional area.
  We may still be interested in seeing logs for the functional area at (e.g.) error level.
• Perform our own filtering in the logging callback.
  This is likely to add an overhead.

It would be useful to be able to set a logging-level override for a functional area. This looks like it could be solvable (mainly) in the LogDispatcher::CheckEnabled method. For example, the single enabling flag for a functional area (in the bitset) could be changed to a logging-level that overrides the global level.

[ethouris]

@maxsharabayko I think this is the time to make the solution provided in #2602 reusable.

[ethouris]

Ok, can you please check if the version submitted for #2674 fixes the problem well enough?