StateSequencer: state machine-based utility for synchronizing multi-threaded tests

I've previously written a CheckPoint class for the same purpose, but while the logs are a little clearer, it's still pretty hard to reason about the expected ordering of events with a CheckPoint-based test.

StateSequencer-based tests should make it clear what the expected order of states should be from the beginning of the test:

sequencer.logicalThread("t1", "t1:block", "t1:resume");
sequencer.logicalThread("t2", "t2:do_stuff");
sequencer.order("t1:block", "t2:do_stuff", "t1:resume");