Migrations access can be optimized

**Is your feature request related to a problem? Please describe.**
#5416 changed how migrations are stored and accessed. How they are currently accessed can be optimized.

**Describe the solution you'd like**
See suggestions in link below

Originally posted by @keith-turner: https://github.com/apache/accumulo/pull/5416#discussion_r2067430572

Start by reviewing issue #5416 and the migration storage and access changes it introduced. Then read the suggestions in the linked pull-request discussion to determine the intended optimization; done means implementing an agreed approach and verifying that migration access still works correctly.

Written by the indexing model from the issue text.