Make locator fetch concurrency configurable

Capping the number of locators fetchers results in a cap in the number
of rollups a single BF process is able to do. Doing more requires
another JVM process, which can get expensive.

Not sure if the work queue for `locatorFetchExecutors` should be made configurable, or unbounded. Currently it is set to: `locatorFetchConcurrency * 5`