More detailed resource usage statistics

[achimnol]

Let's provide three-tuple of statistics for each resource slot type, used / allocated / total, in the admin GraphQL API.

e.g., If an agent has 8 GiB memory and a kernel session is allocated 4 GiB but using only 2.5 GB, we could show a resource gauge like this in a GUI-based console:

NOTE: Later, a single value of absolute "resource slot" in the current code will become a pair of "amount" (absolute real values such as cores and bytes) and "share" (relative virtual decimal values).

• Let agents report detailed stats upon heartbeats.
• Extend the GraphQL API to provide all three used/alloacted/total values
• Deprecate "overbooking" factors -- we are now going to optimize resource limits using real usage traces and via policies
• Update the API documentation