Summary: Add WB API functions to enable better NMT testing
Reviewed-by: dholmes, zgu

Summary: Count number of instance classes so that it matches class metadata size
Reviewed-by: coleenp, acorn

Summary: 1) Fixed MemTrackWorker::generations_in_used() calculation 2) Ensured NMT not to leak memory recorders after shutdown 3) Used ThreadCritical to block safepoint safe threads
Reviewed-by: acorn, coleenp, dholmes, kvn

7151532: DCmd for hotspot native memory tracking
Summary: Implementation of native memory tracking phase 1, which tracks VM native memory usage, and related DCmd
Reviewed-by: acorn, coleenp, fparain