Improved debug tool RunTermQuery.

This tool searches for a term using Query, SpanQuery and direct Term
Vector access. This can be used to diagnose low-level index problems.
Added some getters to Searcher, reduced visibility of instance vars.
Removed calls to set auto-warming of forward indices.

This feature is deprecated; use the external vmtouch tool to improve
caching behaviour instead. See
https://github.com/INL/BlackLab/wiki/Improve-search-speed-using-the-disk-cache
Eliminated SlowCompositeReaderWrapper in Hits.

SlowCompositeReaderWrapper turned out to be the cause of a strange
problem where rare terms were not found even though they occurred in the
index.

Instead of using SlowCompositeReaderWrapper, we now manually loop
through the index segments while gathering Hits. This works properly and
may be faster to boot.

SlowCompositeReaderWrapper is still used in a few other places, but it
appears to work correctly there; we will eventually probably follow the
same approach there as well.