WIP: Add smsc endpoints to HAN

[lrbison]

PR for discussion on how to add smsc endpoints to HAN for local memory exchange.

Motivation: I have a working POC using XPMEM and HAN to increase alltoall exchange speed, but I would prefer to cache the creation of the XPMEM endpoints rather than creating them new each time in the collective.

Questions:

• Where to store the endpoints, and how to properly initialize and deconstruct them.
• What is the difference between mca_coll_han_comm_create_new and mca_coll_han_comm_create?

[lrbison]

Motivating this change is a branch I'm working on to add alltoall (and soon alltoallv) algorithm to HAN.

My testing shows that:

• SMSC End point creation is to slow to be done on-the-fly, so we need a cache somewhere.
• XPMEM SMSC module has the lowest latency, beating out CMA and KNEM, even for copy_to/from, but especially using mapping.

Here are the results I have so far: