monolog: add MwlogHandler

monolog: add MwlogHandler

Introduce a new handler, similar to SyslogHandler, that will be used to
replace udp2log. The main feature of the handler is being able to vary
the 'application name' with each record's channel. This way the channel
can be reconstructed later, e.g. when writing to plaintext files.

See also an overview of the plan here:
https://phabricator.wikimedia.org/T205856#4957430

Bug: T126989
Change-Id: I0c040825b31cd70f197dc8c1d406a103dc0ed1d1