INT-2939 LoggingHandler Improvements

- Don't evaluate expression if the log level is not enabled
- Add mutually exclusive assertions (already protected by the parser
  but not for programmatic configuration).

INT-2944 Document <logging-channel-adapter/>

Add reference sub-chapter.

INT-2939 Polishing - PR Comments

- remove unnecessary cast.
- add setter for level changes.

Polishing

* Change setter to use enum.
* Add getter for current level.
* Rename method to 'createLogMessage'.
INT-2939 Polishing

Add assert for non-null Level.
Merge pull request #754 from garyrussell/INT-2939

* garyrussell-INT-2939:
  INT-2939 Polishing
  INT-2939 LoggingHandler Improvements