pt-kill custom --group-by

Add a --filter to pt-kill like pt-query-digest --filter that will allow the user to create arbitrary keys in the processlist data struct which they can then --group-by. Sponsored work (14304).

--filter is applied to all processlist queries first, no matter their origin (MySQL or --test-matching) before all the match commands. This means, in theory, a complex --filter could replace all the match commands. Otherwise, --filter has no affect on the match commands (unless it does something weird like delete $event->{Info} and --match-info was specified.) The original code for match commands was not changed, so the addition of --filter should be completely transparent and backwards-compatible.

The --filter implementation is identical to that of pt-query-digest. (The code was copy-pasted and tweaked just a little to make it work inside pt-kill.)