Finalize C-API Contract

[planetchili]

This 2.4 will be the final release without a compatibility guarantee in place. This will be the final opportunity to "lock-in" on the C API that forms the interface for the compatibility guarantee we wish to provide. We should take this opportunity to:

• Prune metrics that are not planned to be / were never supported, such as the PSU structured metrics
• Put in place (in introspection) a system for deprecation of metrics etc., or at least a placeholder provision for same
• API versioning scheme and endpoints (might already be implemented by Add DLL Loader Layer to Support Backward Compatibility #398)
• Review PM_STATUS codes, pruning redundancies and add new ones as necessary
• Finalize implementation(s) of pmOpenSession and bring internal variant into public API
• Ensure DLL boundary version compatibility for internal logging functionality (including C-API support for basic log configuration)
• Review the query API and identify anything that should be changed before lock-in
• Review the introspection API similarly