MPDConnection: Replace the executor service with a ThreadedPoolExecutor.
MPDConnection: Replace the executor service with a ThreadedPoolExecutor.

Replace the executor service with a ThreadedPoolExecutor, it will dynamically
allocate the threads as necessary, rather than a FixedPoolExecutor. It will
also allow us to debug how many threads have been used at one time over the
lifetime of the executor, so we can scale back up if someone lets us know
that we're using more than the current maximum threads.

abarisain/dmix,abarisain/dmix,hurzl/dmix,0359xiaodong/dmix,jcnoir/dmix,joansmith/dmix,hurzl/dmix,0359xiaodong/dmix,joansmith/dmix,jcnoir/dmix