Logging improvements

I ran into some issues re: CiviCRM logging:

1. Currently, in many cases, CiviCRM does not indicate a severity level when logging a message. Thus many events are logged at info level.
2. When passing log messages thru to Drupal's watchdog() logger, debug level is typically used, even for a fatal error. There is no attempt made to map the level appropriately.
3. Some of the CRM_Core_Error methods - e.g. CRM_Core_Error::debug_var() - don't have an argument for severity level - but we could/should add one.
4. When providing a severity level to "old school" CRM_Core_Error calls, we also have the option of "upgrading" these to instead be Civi::log() calls.

I would be curious to hear anyone's thoughts, but it seems fairly straightforward to make some big improvements here.