Spine's batch dropping seems erroneous.

[frankmcsherry]

Spines drop batches when their advance_frontier reaches the empty set. This has a few flaws, however:

1. The batch bounds are used by read_upper() to indicate how far along the trace has progressed. In principle one can announce that they don't care about times, but still read from read_upper() for some reason.
2. Once batches are dropped, new batches can still land in the trace. The frontier will not advance again, and so these batches will probably just be held until the trace is eventually dropped.

I'm not aware of settings in which a trace has its frontier advanced to the empty frontier, but is not itself dropped. We could plausibly just remove the eager dropping of batches, and try and reconstruct which mode of execution holds on to more memory than it is supposed to, and track down whichever villains are holding on to traces the cannot read out of.

[frankmcsherry]

Fixed with #231