Investigate adding a config option for setting thread pool size in InstanceOperations

This is a follow on to #5102 based on [this](https://github.com/apache/accumulo/pull/5102#discussion_r1866555388) comment. We should look into setting a lower default and possible configuration option for max number of threads for when we create thread pools to query multiiple servers at one time in InstanceOperations. The current max is 256 could be too many for some clients.

Start by reviewing InstanceOperations and the discussion in issue #5102 that motivated this follow-up. Determine how thread pools query multiple servers, then assess a lower default and a configuration option for the maximum thread count; done means the configuration approach and appropriate limit are defined.

Written by the indexing model from the issue text.