Add metadata to local_channels table

[pm47]

(First commit is a refactor/cleanup and can be reviewed independently).

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.

[pm47]

@sstone @t-bast the PR isn't finished but I'd like to have your feedback before I go further. I did start by implementing this in the audit db, before changing for this approach.

[t-bast]

That sounds reasonable, these are good reasons to go down that route.
As long as we use separate, optional columns and not modify the core ChannelData structures let's try and see where it takes us.

[codecov-io]

Codecov Report

Merging #1724 (b46a27f) into master (afa378f) will decrease coverage by 0.07%.
The diff coverage is 81.92%.

@@            Coverage Diff             @@
##           master    #1724      +/-   ##
==========================================
- Coverage   86.30%   86.22%   -0.08%     
==========================================
  Files         151      151              
  Lines       11701    11781      +80     
  Branches      501      498       -3     
==========================================
+ Hits        10098    10158      +60     
- Misses       1603     1623      +20     

Impacted Files	Coverage Δ
...r-core/src/main/scala/fr/acinq/eclair/Eclair.scala	54.74% <ø> (ø)
...ain/scala/fr/acinq/eclair/db/pg/PgChannelsDb.scala	85.13% <60.86%> (-11.17%)	⬇️
...main/scala/fr/acinq/eclair/db/DbEventHandler.scala	90.32% <84.37%> (ø)
...a/fr/acinq/eclair/db/sqlite/SqliteChannelsDb.scala	95.94% <95.65%> (-0.36%)	⬇️
...ir-core/src/main/scala/fr/acinq/eclair/Setup.scala	70.83% <100.00%> (ø)
...c/main/scala/fr/acinq/eclair/db/pg/PgAuditDb.scala	98.97% <100.00%> (ø)
...cala/fr/acinq/eclair/db/sqlite/SqliteAuditDb.scala	99.06% <100.00%> (ø)
.../scala/fr/acinq/eclair/payment/PaymentEvents.scala	97.61% <0.00%> (-2.39%)	⬇️
...main/scala/fr/acinq/eclair/router/Validation.scala	90.76% <0.00%> (-1.54%)	⬇️
...nq/eclair/blockchain/electrum/ElectrumClient.scala	73.52% <0.00%> (-0.37%)	⬇️
... and 5 more