Core: reuse Lucene's TermsEnum for faster _uid/version lookup during indexing

The TermsEnums used for lookup have highish cost to init, so if we
reuse them we may be able to stop using bloom filters.  I ran some bulk
update performance tests, showing that turning off blooms and reusing
the enums gets close to the same performance as master (using blooms
and not reusing the enums).

Closes #6212