NRTCachingDirectory shouldn't log to stdout [LUCENE-7082]

NRTCachingDirectory declares `final boolean VERBOSE = false` and then checks this value when determining whether to write additional logging to stdout or not.

Since this is only useful during development, we should change all of the statements to use proper debug logging that can be turned on/off via configuration in production.

---
Migrated from [LUCENE-7082](https://issues.apache.org/jira/browse/LUCENE-7082) by Mike Drob (@madrob)

Locate the NRTCachingDirectory class and inspect every statement guarded by the VERBOSE constant that writes to stdout. Determine the project's existing debug-logging convention before changing those statements, and verify that the directory no longer writes development logging to stdout while logging can be enabled through configuration.

Written by the indexing model from the issue text.