Extract tx-log ingestion logic out of block-store

[karlfloersch]

Is your feature request related to a problem? Please describe.
Currently block-store.js handles way more than it should. It ingests transactions from the tx-log, triggers the leveldb-sum-tree logic, and generates history proofs. This is too much.

Describe the solution you'd like
The first step in cleaning up the block-manager is extracting the tx-log ingestion logic. tx-log ingestion should be a separate file which one can call, pass in a db and then wait util that db is populated with transactions from a particular block. This is a very clean separation of concerns and should be easy to pull out.

Relevant code: https://github.com/plasma-group/plasma-chain-operator/blob/master/src/block-manager/block-store.js#L365-L452