Added more to locked object test.  Now try to write to variable as
well as read it.  Test currently fails.
Locked object test previously hung on read after write.  This was
because I incorrectly completed write commits: I wouldn't overwrite
the write event's write lock on an object because I was accidentally
comparing a uuid to an event, rather than a uuid to a uuid.  Fixed
comparison, plus unified usage of write_lock_holder to be an event
rather than uuid.  Locked object test still fails, but now it fails
because reading stale value in a write event.
Fixed minor bug: was not reading from dirty cell after write, but before event committed.
Added a test to ensure can perform parallel reads on single object.
Minor renaming in test file.
Added a test to ensure that older events correctly preempted younger events.  It currently deadlocks.  Must fix.