Use LogLevelMapping to convert JSON wire protocol name to Level instance.

Without this, the java bindings fail to parse the "DEBUG" log level,
which is valid in JSON wire protocol but not a recognized Java log level
name.
Clean up internal Firefox logging API.

* Removed redundant code
* Use named loggers so it's easier to find log message origins
Add explicit API for configuring log prefs rather than forcing users to rely on
magic capability strings.
Catch driver start-up failures.