Just like {@link #forceMergeDeletes()}, except you can specify whether the call should block until the operation completes. This is only meaningful with a {@link MergeScheduler} that is able to run merges inbackground threads.
NOTE: if this method hits an OutOfMemoryError you should immediately close the writer. See above for details.
NOTE: if you call {@link #close(boolean)}with false, which aborts all running merges, then any thread still running this method might hit a {@link MergePolicy.MergeAbortedException}.