Improve ordhook performance & maintainability

[smcclellan]

Several tickets can be grouped here to improve the performance & maintainability of ordhook. The title of this will get more specific as tickets are added.

• Allow observers.sqlite to be configured in a separate working directory #274
• Add Prometheus metrics #282
• Provide support for Testnet #292
• guarantee db write atomicity between brc20.sqlite and hord.sqlite #288
• Improve resiliency regarding underlying bitcoind  #289
• Increase code coverage to 75% #352
• add codecov reports to CI #341
• posting brc20 activity without --config-path #337
• ordhook does not recover predicate upon restart #338
• wait for bitcoind to be ready on boot #318
• Add flag to stop service when reaching bitcoin chain tip #361

[rafaelcr]

Marking as complete as we will take care of remaining tasks individually