Cleaner cluster termination when using MPIManager

[amitmurthy]

A few related issues:

• warnings printed when using MPI transport
• different finalization procedures between when using MPI_ON_WORKERS, MPI_TRANSPORT_ALL, and TCP_TRANSPORT_ALL. Implement a standard "close" function.
• warning printed by Julia event loop when using MPI_ON_WORKERS

[psanan]

I noticed similar issue on the cray system (I assume that the problems above are observable on any system but I can provide more info and/or test any fixes if that'd be helpful). Parallel jobs terminate with the following:

WARNING: Forcibly interrupting busy workers
WARNING: Unable to terminate all workers