Finer MINIMART_TRACE control over tracing.
Make ground async-channel always available, rather than dynamically
scoped to run-ground. This is important because (spawn-timer-driver),
for example, is called outside run-ground and spawns a thread that
communicates with ground via the ground async-channel. When we move to
running multiple grounds at once, we will have to revisit the
parameterization scheme here.
Make timer driver use send-ground-message rather than changing gestalt.