feat: support re-sending events to event observers across restarts

Record events to be sent to event observers in a new sqlite database so
that in the event that the node is killed before successfully sending,
they can be re-sent on restart.

Author: obycode

CHANGELOG.md

@@ -18,6 +18,7 @@ and this project adheres to the versioning scheme outlined in the [README.md](RE
     - `get-block-info?` removed
 - Added `/v3/signer/{signer_pubkey}/{reward_cycle}` endpoint
 - Added optional `timeout_ms` to `events_observer` configuration
+- Added support for re-sending events to event observers across restarts
 
 ## [2.5.0.0.7]