Add Monolog formatter that mimics legacy log output

Having a log formatter for the Monolog stack that mimics the legacy wf*
logging function output will ease the transition for users wishing to
use Monolog who have tooling that expects the legacy log formats.

Bug: T845
Change-Id: I06295ccc4b068c61d7971024213366004b69c03d
Allow limiting Monolog output using legacy settings

Add $useLegacyFilter option to MWLoggerMonologHandler constructor that
will use MWLoggerLegacyLogger::shouldEmit to decide if a given log
message should be emitted.

Bug: T845
Change-Id: If311308faad35348fdc7e85155a1bc16bbf75c85
Merge "Add Monolog formatter that mimics legacy log output"
Merge "Allow limiting Monolog output using legacy settings"