Support split OTR message on receive

This has been observed in bitlbee where a full OTR message is split in
different PRIVMSG so now this patch adds a support to check for
fragmented OTR message and reconstruct it before passing the message to
libotr.

Reported-by: Micah Anderson <micah@riseup.net>
Reported-by: Jacob Appelbaum <jacob@appelbaum.net>
Signed-off-by: David Goulet <dgoulet@ev0ke.net>