Add support for the udreg registration cache and dynamics on XE/XK/XC.

To support the new mpool two changes were made to the mpool infrastructure:

 1) Added an mpool flag to indicate that an mpool does not need the memory
    hooks to use the leave pinned protocols. This flag is checked in the
    mpool lookup.

 2) Add a mpool context to the base registration. This new member is used
    by the udreg mpool to store the udreg context associated with the
    particular registration. The new member will not break the ABI
    compatibility as the new member is only currently used by the udreg
    mpool.

Dynamics support for Cray systems makes use of the global rank provided by
orte to give the ugni library a unique rank for each process. Dynamics
support is not available under direct-launch (srun.)

cmr=v1.7.4