Add BackfillStatus and Improve StartingVersion Selection on (re)start

[dermanyang]

Purpose

Add new backfill_status Enum to BackfillProcessorStatus. The enum can either be InProgress or Complete. This will help the get_starting_version function decide whether or not to respect the latest checkpoint in the backfill processor status table or to just use config.starting_ver / genesis. Read more in the design

Technical Overview

Mostly the addition of logic into the starting_version.rs file. Otherwise, some convenience functions implementing ToSql and FromSql for the BackfillStatus enum.

New logic will mark BackfillStatus as Complete when lst_success_version >= backfill_end_version and as InProgress otherwise.

Testing

Added new dependency on the testing framework (!) which allows us to use a mock postgres instance so we can test the starting_version logic by manipulating the persisted state.

Added 4 test cases to lock the intended behavior:

1. regular processor start with no checkpoint (empty DB)
2. regular processor (re)start with checkpoint
3. backfill processor start with a Complete checkpoint
4. backfill processor with an InProgress checkpoint

Please note I tried to reduce the setup code repetition in the tests but couldn't fix the connection refused error within my time box 🤷‍♂️

Also tested the above scenarios manually. Works as expected.

[yuunlimm]

approved with a minor comment

[yuunlimm]

nit: should we consider adding a test case where the configuration has a starting_version explicitly set?

[dermanyang]

added! thanks