Javadoc
Translates fields as
needFlushWrites and
needFlushDeletes in a set of requirements as checked
by reader threads. This is commonly invoked by a single thread (so no contention on this method
is expected) but it needs to expose a consistent view of the written fields to
#refreshReaders().
This is normally not invoked in parallel by multiple threads as the backend design allows a single working thread
per index, but it could be invoked concurrently when streaming work is being applied (when a MassIndexer is
running). Note that multiple threads invoking this in parallel might result in skipping some sequence numbers
but that's not a problem.