1. Search backwards through our tags to find the most recent,
best matching id.
2. Replay tags after the found id to adjust for any pending changes.
In theory this should work in controlled cases, but there are a lot of
corner cases around grows and shrinks. Tests are written, and failing,
but I think it may be simpler and more efficient to implement this in a
single pass, with tighter assumptions about what grow/shrinks are
allowed.
Christopher Haster
c0ee405cf2