Mempool - Optimise Angor indexing

[nostrdev-com]

The current implementation of the indexing process means that it will run each time indexing starts, fetch all the transaction ids and then check if each of these transactions has been indexed by querying a separate database, Angor Blocks Repository.

A possible solution that would eliminate regular, redundant querying is to store two flags, one with the latest block that was indexed and one to verify that all of the blocks up until the latest verify blocks have been indexed. The second flag is needed because in my experience blocks are not always indexed in the consecutive order.