We should never open an IndexInput when an IndexOutput is still open

I modified MockDirWrapper to assert this (except for
segments_N/segments.gen, where it's expected), and, it uncovered a
couple of places involving NRT readers where we open a shared doc
store file that's still open for writing.

First, if you install a merged segment warmer, we were failing to
force the merge of the doc stores in this case, thus potentially
opening the same doc stores that are also still open for writing.

Second, if you're actively adding docs in other threads when you call
IW.getReader(), the other threads could sneak in and flush new
segments sharing the doc stores. The returned reader then opens the
doc store files that are still open for writing.