Margo RPC instrumentation mechanism
- turn on margo diagnostics to produce output
- reports min/max/avg for each rpc issued
- identifiers for each rpc recorded include the ancestry of rpcs that triggered them
- relies on user-defined Mercury header space and Argobots thread-local keys
Ready to merge (and expand later).
Rudimentary documentation can be found in doc/instrumentation.md, which is linked from the top level README.md.