Some users reported that mod/chat was silently dropping some messages. The
user's browser may not be able to keep up with the rate at which he submits
chat messages. This update keeps things sane.
Merged from MOODLE_16_STABLE
Author: Donal McMullan <donal@catalyst.net.nz>
+ we are now more conservative in the fields it asks from the db
+ also more conservative with upates to chat_users
+ messages are now added via POST requests instead of GET