Major Description
I think NRTManager can be drastically simplified by moving all of its
logic into a new reopen thread class. All logic for waiting for a
specific generation and reopening at different rates would live in
this class.
This would fully decouple the "wait for generation X to be visible"
from which particular ReferenceManager impl you're using, which would
make it possible to use the controlled consistency approach of
NRTManager with any managers (e.g. SearcherTaxonomyManager).