Reduce the cost of calling LegacyLogger::debug() when there is no debug
log enabled (the normal production case) from 0.8µs to 0.2µs, measured
locally, by duplicating some of the logic from log() and shouldEmit() to
derive a constant "minimum level".
I also introduced constants for the integers, to avoid unnecessary
lookups in self::$levelMapping, and I introduced $this->isDB, to avoid a
hashtable lookup in log(). I fixed a typo in a comment, and removed
"@return null", which was confusing PHPStorm.
Change-Id: I9fc37b8062ff22f85feda9a05821e3d8c9688519
a4ae47a7b7