udp2log: demux now filter groups stricly

When relaying the fatal and exception log group, we only want to match
two groups:

- exception
- fatal

Ori wrote a patch in MediaWiki core that let us optionally log the
exception in json format under the 'exception-json' debug log group. The
previous regex filtering would thus match both exception groups and
would lead to a double count of exceptions. Adding a trailing whitespace
(the separator between group name and payload) in udp2log protocol would
make use filter groups explicitly.

Change-Id: I422e592ae360bf03082306bff1e00338edd1292c
Merge "udp2log: demux now filter groups stricly" into production