OSHMEM: sshmem verbs: allocate memory at fixed address

Use experimental verbs to allocate memory at fixed base
virtual address.

verbs will disqualify itself if shared_mr is disabled
or not supported and it is impossible to allocate memory
starting at fixed base virtual address.

verbs contig pages allocator did not guarantee fixed va, now it does.
(cherry picked from commit fd77ebd4525e9e0c1a3ab1c4966bf31aa45251b4)

Apply Jeff`s comments

Update with Jeff commits
(cherry picked from commit open-mpi/ompi-release@4dc487fc3d6353022a68717f17a6cda8fa15877d)