Change DB stucture, improve LMC behaviour

Please, backup your DB before performing any testing with changes in code.

Introduce new DB structure and DB migration mechanism.
The structure despite being unintuitive allows better maintainability,
**performance** and data integrity.

The new way is to replace original message, while keeping the original in
LMC messages, this way we can preserve full edit history
and avoid wasteful JOINs on every scroll up.

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` column as well.

Performance boost is noticeable.

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