Use the leaky bucket algorithm to rate limit irc messages.

The default values were suggested by an operator of libera.

We don't rate limit the login process, nor parts and quits. However, if you
paste a bunch of text and then part a channel, you will be spammed with a
bunch of "no such nick/channel" error dialogs. I tried to work around this,
but the alternative just makes irc unresponsive until all the pasted messages
are sent. That said, other messages are still delayed while these errors
dialogs are slowly popping up.

Testing Done:
Lots

Bugs closed: PIDGIN-11089

Reviewed at https://reviews.imfreedom.org/r/524/