mpi/pmix: one more I/O refactoring prior to comm refactoring

- Provide ability to initialize I/O engine without knowing fd this will
be used when we will be sending connection request using SLURM protocol
and queuing all other send's waiting for connection to be established.
- Simplify some I/O API and I/O internals.

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