Refactor: separate Collection API commands from Overseer and message handling logic

Collection API command execution happens in Overseer. The code dealing with Overseer specific abstractions (Collection API queue management, executing threads etc) is mixed with code implementing the Collection API commands.

The goal of this ticket is refactoring the Collection API code to abstract anything that is related to how the Overseer executes the commands, in order to enable a future ticket (SOLR-15146) to introduce a distributed execution mode for the Collection API (and keeping the changes limited).

This ticket does not introduce any changes regarding how the Collection API commands run in the Overseer. It is only refactoring the call chains to allow a future separation.