MCA: Per-RFC, add support for performance variables

This commit adds an API for registering and querying performance
variables (mca_base_pvar) in the MCA base. The existing MCA variable
system API has been updated to reflect the new API: MCA variable
groups have performance variables, and new types have been added (double,
unsigned long long) to reflect what is required by the MPI_T
interface. Additionally, the MCA variable group code has been split
into its own set of files: mca_base_var_group.[ch].

Details of the new API can be found in doxygen comments in the header:
mca_base_pvar.h.

Other changes to the variable system:

 - Use an opal_hash_table to speed up variable/group lookup.

 - Clean up code associated with MCA variable types.

 - Registered performance variables are printed by ompi_info -a. In the
   future an option should be added to control this behavior.

Changes to OMPI:

 - Added full support for the MPI_T performance variable interface.
Add example performance variables to ob1: unexpected message queue length, posted receive length
ignore unavailable components when registering
Add profiling function definitions for MPI_T and add a missing type into mpi.h
mpit: fix behavior when returning strings