Monitor to support search concurrency

### Description

There is currently no way to provide an executor to the index searcher used internally by monitor.

There may be value in parallelizing execution, but besides exposing the ability for users to provide an executor like we do with ordinary search, a refactor is needed to properly support inter-segment concurrency in monitor.

The issue names the index searcher used internally by monitor and ordinary search's user-provided executor as the starting points. Compare those paths first, then define the scope of exposing an executor and supporting inter-segment concurrency; done means the monitor behavior and concurrency requirements are clearly established.

Written by the indexing model from the issue text.