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.

[wenduwan]

I verified that with this PR, HAN's info keys are properly propagated to the subcommunicators.

[bosilca]

The need for this PR arise because the info code, lacking documentation, was misunderstood and capabilities were mistakenly removed. For the sake of future OMPI developers we need to properly document these changes.

[bosilca]

This is a major change, it deserves better documentation.

[devreal]

Fixed

[bosilca]

Move these into a header file and make them always inline.

[devreal]

I'm hesitant about this since there are no other implementations in info.h. The call to opal_info_dup_impl will be a jmp so there is little to gain from inlining these.

[bosilca]

A call into a function that does a simple jmp ? I don't even see the interest of having these functions, with a weird name, instead of just using opal_info_dup with all 3 arguments.