refactor/feature: split up indexing/contract/db logic + make watcher functional again

[luketchang]

CommonIndexer/HomeIndexer: Interface by which chain-specific homes/replicas bubble up event data (each chain will have a different implementation/library used retrieve this data which is why we need a trait to provide common interface). Bubbles up data to a ContractSync, which takes that data and stores it in agent's db. ContractSync: Drives syncing between agent db and on-chain data in loop. Continuously calls out to an indexer to retrieve event data and stores it in agent's db. CachingHome/CachingReplica: Both contain a sync function which spawns a new ContractSync to keep db up-to-date.

Closes #870

[luketchang]

Documentation:

• Indexer traits: https://github.com/celo-org/optics-monorepo/pull/925/files#diff-6bd64866866ea21992b5ba0925c8dcaa2efca43e5d5409d369e4bec27f85be4bR1-R8
• ContractSync struct: https://github.com/celo-org/optics-monorepo/pull/925/files#diff-8915f2b78fa646fab4b83ec99793b39e5c908ed3cb74af8dc49f403ea5d40b94R16-R20
• HomeEvents/CommonEvents: Homes and Replicas abstractions, which switch over chain-specific contract methods are retained and implement Home and Replicas.CachingHomeandCachingReplicaadditionally provide event data (updates, messages) and thus implementHomeEventsandCommonEvents` (chain agnostic since they are just calls to db).