vendor/session: Implementation of MPIR_Session_reinit_impl.

Sanity checks that make this function return an error upon failure
- PMIx 4.2.4 or higher must be used
- The count argument must be equal to number of existing sessions
- The world model must not be initialized (no MPI_(thread)_Init)
- Rank 0 must never leave
- A rank must not be marked to leave if nleave is zero
- All sessions must be malleable
- (No conflict with async spawn - to be implemented)

Steps that this function performs:
1. Call prep_fn (if provided) for all sessions in array
2. Finalize all sessions in array
3. Synchronize staying processes; wait until leaving processes are gone
4. (Return any free nodes if user requested this - to be implemented)
5. Initialize one new session
6. Call resume_fn (if provided) for new session