Cleanly stop periodic callbacks in Client (#2705)
Cleanly stop periodic callbacks in Client (#2705)

Previously we did this only in the asynchronous code, which left a gap
during which a heartbeat could sneak out.  Now we call it explicitly at
the beginning of the synchronous close command.
blaze/distributed,dask/distributed,dask/distributed,mrocklin/distributed,dask/distributed,mrocklin/distributed,mrocklin/distributed,dask/distributed,blaze/distributed