Add back fast path for non-gappy syncs (#17064)

PR #16942 removed an invalid optimisation that avoided pulling out state
for non-gappy syncs. This causes a large increase in DB usage. c.f.
#16941 for why that optimisation was wrong.

However, we can still optimise in the simple case where the events in
the timeline are a linear chain without any branching/merging of the
DAG.

cc. @richvdh