* rename `Auditor` to `DbEventHandler`

Move it to the `db` package (it was in the `payments` package for
historical reasons but doesn't deal only with payment anymore).

Better typing for channel lifecycle event.

* add meta info to local_channels table

Here is the rationale for implementing channel metadata as additional
columns in the `local_channels` table of the `channels` db, as opposed
to a dedicated `channel_metadata` table of a `audit` db:

1) There is a migration to do (in the `local_channels` table, no less!),
but it's just a table migration, as opposed to a data migration, if we
had to populate a new table in a separate database.
2) We don't need to worry about creating a new metadata line when a new
channel is created (compared to doing add-or-update stuff). It's only
_updating_ optional columns in a best-effort manner.
3) We don't need to worry about inconsistencies between two tables
located in two separated databases (that's a big one).
4) We may want to use the metadata during operations, not just for
audit purposes. For example to close channels that have stayed unused
for a long time.
5) The audit db is an append-only log of events and shouldn't be used
for anything else. There is no `UPDATE` sql statement in
`*AuditDb.scala`. The `channel_metadata` would break that heuristic.