Add a context class for tracking what a thread is doing. This is the
first step towards generic instrumentation for frwlock contention.
Add context engine status entries
refs #146 Rename context_status_id -> context_id, sprinkle some
context's around the code
refs #146 Add context rows for promotion, add more context sections to the
cachetable
Merge branch 'diagnostics' into scoped_malloc

Conflicts:
	util/frwlock.cc
	util/frwlock.h