* Fix joins to many channels
If you have enough channels that the 512 byte message limit on the JOIN
message is hit then limnoria was losing the channel that put it over the
limit and not including it in the next JOIN message. This resulted in
losing one channel for every JOIN message that pushed us over 512 bytes.
We fix this by generating the JOIN message immediately after resetting
the channels list to ensure we include the channel that pushed us over
the limit. Then the next time through our JOIN msg construction we'll
add subsequent channels without forgetting the one that pushed us over.
* Add test for channel join lists
This adds a test for the issue that is fixed in the previous commit. We
ensure that when JOINs are split over multiple messages we JOIN to all
channels that were part of the input list and don't forget any of them.
