Add a function for syncing blocks from Zebra's non-finalized best chain to a ReadStateService in another process

[arya2]

Motivation

We want the ChainTipChange struct to work in a separate process so it can be used in zebra-scan and/or the indexer.

We also want to have access to all of Zebra's best chain via the ReadStateService from another process, including blocks in the non-finalized portion of Zebra's best chain.

Note: Accessing the finalized portion of Zebra's best chain should only require a ReadStateService initialized with a read-only ZebraDb using the same cache directory (except maybe when Zebra is configured to use an ephemeral state).

Design

Add a function in zebra-rpc that:

• Polls the getbestblockhash method to check if a process has the latest best chain tip in Zebra
• Calls the getblock method to sync any blocks in Zebra that are missing in the new process
• Infers when there's been a chain fork/reorganization and re-syncs all of the blocks past the finalized tip from Zebra
• Sends new blocks to the LatestChainTip channel used by ChainTipChange

This function could returns a ChainTipChange struct, or accept a ChainTipSender.