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.
Define config var earlier

Need to define the config var earlier so it can be used
when configuring LocatorFetchThreadPool
Merge pull request #323 from dlobue/configurable_locator_fetch_concurrency

Make locator fetch concurrency configurable