mpi/pmix: Rework communication framework (step 1).

- Introduce data structures reuse to avoid extra malloc/free overhead.
- Introduce direct TCP connection notation (disabled for now)

This commit is intended for testing SLURM protocol portion of the code
to see that nothing is broken.
Direct communication will be enabled in the next commit.

Signed-off-by: Artem Polyakov <artpol84@gmail.com>