consider sending set by/at on BMASK burst

[jesopo]

My understanding is that not sending these was a space saving mechanism but we have more bandwidth now and it does actually lead to two real-world issues that I've observed frequently:

• users cannot find who to message to contest their ban
• ops forget why a ban was set and can't ask an unknown setter (solved by bantracker bots)

I've considered how one would best change the BMASK format to include these params and there's only two solutions that come to mind:

• a special delimiter character, much like banforwards
• space-separated and the receiving end batches params in to groups of 3

I took an average of hostmask lengths in #freenode (1549 users) and found 40.2 bytes, so both of these solutions would add, on average, 52.2 bytes per mask (10 for ts, 52.2 for setter, 2 for delimiters)

you could, somehow, do deduplication of setter hostmasks; i am not sure of a good way to do this