Allow max concurrency limits configuration on the indexer

**Is your feature request related to a problem? Please describe.**
allowing max concurrency config will help in running index on smaller cpu machines and also free up some bitcoin rpc bandwidth for other uses.

The issue names no files, tests, or entry points; start by locating the Go indexer and where it makes concurrent Bitcoin RPC calls. Done means the indexer accepts a configurable maximum concurrency and can run with that limit on smaller-CPU machines while preserving indexing behavior.

Written by the indexing model from the issue text.