With this commit, slixmpp-omemo now reads the ratchet chain length,
(both receiving and sending), that we should track to know when to send
a heartbeat message.

This allows us to signal other devices that we are still active and
listening. Some clients will stop encrypting to us if we haven't replied
for a certain number of messages.

The current 0384 spec (0.7) says we should send a heartbeat message at
least once this number goes over 53 (fair dice roll). It doesn't say
when a client may/should stop encrypting to us, or what it should do at
all once we go over 53.

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>