Change DB stucture, improve LMC behaviour

**Please, backup your DB before performing any testing.**

Introduce new DB structure and DB migration mechanism.

Now message LMC messages are interconnected with original messages,
this way we have fast access to last (hence correct) applicable edits,
as well as reference to the original message from the any edit (in case of chained edits).

Change the way LMC messages are being displayed. Now we check if we
can replace a message from current buffer. If we don't have a message in
the buffer, it might've been lost, but we can still display it as a
new message.

Index `timestamp`, `to_jid`, `from_jid` columns to improve performance.

Further information available here:
profanity-im#1893
profanity-im#1899
profanity-im#1902