[5.0.x] Fix the info subscriber mechanism and hidden info keys

[devreal]

PR #9246 was overzealous in dropping keys, which broke MPI_[Comm|Win|File]_set_info. Also, @wenduwan noted in #11823 that info keys are not properly propagated to subcommunicators in HAN. The root cause is that keys are not kept if there were no subscribers.

This PR makes the following changes:

• Make sure info keys are always stored in the info object, independent of whether there are subscribers.
• If there are no subscribers we mark the keys as internal (previously this was done using a IN prefix to the key, now it's a flag). Internal keys are not handed back to the user in MPI[Comm|Win|File]_get_info.
• Remove the function for explicitly managing the reference count and removing unreferenced info keys. We still track whether info keys are referenced and keys that are not referenced are treated like internal keys.

Cherry-pick of #12498 to v5.0.x

Signed-off-by: Joseph Schuchart [email protected]
(cherry picked from commit 158e54c)