Use multiple threads to apply deletes and DV updates

Today, when users delete documents or apply doc values updates, IndexWriter buffers them up into frozen packets and then eventually uses a single thread (BufferedUpdatesStream.applyDeletesAndUpdates) to resolve delete/update terms to docids. This thread also holds IW's monitor lock, so it also blocks refresh, merges starting/finishing, commits, etc.

We have heavily optimized this part of Lucene over time, e.g. LUCENE-6161, LUCENE-2897, LUCENE-2680, LUCENE-3342, but still, it's a single thread so it can't use multiple CPU cores commonly available now.

This doesn't affect append-only usage, but for update-heavy users (me!) this can be a big bottleneck, and causes long stop-the-world hangs during indexing.

I have an initial exploratory patch to make these lookups concurrent, without holding IW's lock, so that when a new packet of deletes is pushed, which happens when we flush a new segment, we immediately use that same indexing thread to and resolve the deletions.

This is analogous to when we made segment flushing concurrent (LUCENE-3023), just for deletes and DV updates as well.