Holding on to a trace with physical/logical compaction to the empty frontier stalls compaction

[antiguru]

(Placeholder, @antiguru to add more info+repro)

While working on MaterializeInc/materialize#18766, we noticed that allowing physical and logical compaction of a trace to the empty frontier prevents compaction, even if other owners of the trace eventually allow compaction. It is required to repeatedly allow compaction with a sensible frontier (i.e., read_upper followed by allowing compaction) to enable compaction of the spine.