feature: add EthereumReplicaIndexer and have watcher spawn indexing tasks for replicas

[luketchang]

• Creates EthereumReplicaIndexer which is essentially the same as the HomeIndexer but just syncs updates
• A lot of repeated code but there is a refactor in progress that I plan on putting up after we actually get a watcher up and running (faster to just reuse the existing pattern for now and refactor later)
• Watcher now spawns replica sync tasks so that it can retrieve proper info

Part of #899