Core: don't close/reopen IndexWriter when changing RAM buffer size

Today we close/reopen IW when we change the RAM buffer but that's
costly because it means the next NRT reader is a full reopen.  The RAM
buffer size setting is a live one in IndexWriter, even if there are no
buffered docs in RAM when you call it.

Separately it would be nice if Lucene let you manage a "reader pool"
that could outlive individual IW instances ...

Closes #6856