Allow MWLoggerMonologHandler to take prefix from channel

Allow MWLoggerMonologHandler to take prefix from channel

Make MWLoggerMonologHandler replace a prefix of "{channel}" with the log
event channel when formatting an event for a udp2log stream. This allows
a simpler configuration for a typical Monolog configuration where
a separate file is desired for each logging channel that is emitted to
udp2log. One handler can be shared by multiple channels.

Change-Id: Ie2b24ba02b281b8d8cf2ad58d28874c710e1e2e5