Item12104: Add support for eachEvent multilevel

logger->eachEventSince will now accept an array of requested levels.  It will
only process a single file for now.  Still needs support for merging log
levels from multiple files.  If the requested levels span more than one
file, only one file is processed.

Also eachEventSince now returns the level as the last value in the array.

LogDispatchContrib doesn't support this yet, so the unit tests all fail
for the LogDispatch logger.

git-svn-id: http://svn.foswiki.org/trunk/core@16436 0b4bb1d4-4e5a-0410-9cc4-b2b747904278