Reinstate the new signal handling; revert the revert.

Reinstate the new signal handling; revert the revert.

The fix for high CPU usage was not doing wakeup() all the time for
new data; not only can seemingly sending the signal be expensive,
but we sent it while still holding the mutex the awakening thread
needed, so it was extra-bad.

We're seemingly down to a reasonable amount of CPU usage again now,
but still with good non-racing semantics.