Major Description
This is a simple helper class I wrote for Lucene in Action 2nd ed.
I'd like to commit under Lucene (contrib/misc).
It simplifies using & reopening an IndexSearcher across multiple
threads, by using IndexReader's ref counts to know when it's safe
to close the reader.
In the process I also factored out a test base class for tests that
want to make lots of simultaneous indexing and searching threads, and
fixed TestNRTThreads (core), TestNRTManager (contrib/misc) and the new
TestSearcherManager (contrib/misc) to use this base class.