IndexWriter does not do the right thing when a Thread is interrupt()'d

Spinoff from here:

http://www.nabble.com/Deadlock-with-concurrent-merges-and-IndexWriter--Lucene-2.4--to22714290.html

When a Thread is interrupt()'d while inside Lucene, there is a risk currently that it will cause a spinloop and starve BG merges from completing.

Instead, when possible, we should allow interruption. But unfortunately for back-compat, we will need to wrap the exception in an unchecked version. In 3.0 we can change that to simply throw InterruptedException.