Add command queue prepend, prioritize PING/PONG

Add command queue prepend, prioritize PING/PONG

Add optional flag to putRawLine and putCmd to put any given lines or
commands to the front of the queue (prepend instead of append).

Enable queue prepend for sending PINGs and PONG replies.  This fixes
the issue where Quassel will ping-timeout when trying to send huge
messages (e.g. 40 lines at once), or multiple commands.

Unfortunately, some IRC servers don't behave properly so Quassel may
still get disconnected.  Hopefully other servers will get fixed soon.

Add more documentation, of course.  Document all the things!

(cherry picked from commit 45a0d954542db252ceb62b61243ee5c2253383de)