module indexer introduces a extensible architecture for indexing the
chain. It provides a base class which handles syncing with the chain,
handling re-orgs, interruptions, dynamic toggling, etc. TXIndexer
and AddrIndexer are provided for indexing transactions and addresses,
using the same flags as before i.e --index-tx and --index-address.
Indexes are stored in a different database and can be maintained
independently of the chain.
