Add AsyncFSDirectory to work around Windows issues with NIOFS (Lucene 5.0 only)

On LUCENE-4848 a new directory implementation was proposed that uses AsyncFileChannel to make a sync-less directory implementation (only needed for IndexInput). The problem on Windows is that positional reads are impossible without overlapping (async) I/O, so FileChannel in the JDK has to syncronize all reads, because they consist of an atomic seek and atomic read.

AsyncFSDirectoty would not have this issue, but has to take care of thread management, because you need a separate thread to get notified when the read is done. This involves overhead, but might still be better than the synchronization.