Add a misconfiguration diagnostic or highlight mismatched resource.mb vs -Xmx

It is possible to misconfigure a pure-java MR application by setting mapreduce resource size to 1536Mb, while leaving the -Xmx value at the default 512Mb.

The AM UI can track over-all memory usage of an app either via Runtime.maxMemory() or via the process watcher in YARN to report the actual memory used by the application.

MR does not fail even when the memory -Xmx is set lower than the YARN allocations, but it spills & GCs instead to keep it with in the memory sizes.

This is sub-optimal and would make sense for YARN to highlight in the AM UI or JHS UI for tuning/debugging purposes.