We should not use depth in the sync API

*This issue was originally created by [**@neilalexander**](https://github.com/neilalexander) at .*

Currently the sync API uses the event depth when inserting events into the topology, which we should really not do since depth is deprecated.

(Incidentally, we probably need to come up with some better way to maintain topological ordering with inserts.)

Trace the sync API path that inserts events into the topology and identify where event depth is used. Review the surrounding topology-ordering logic and existing tests, if present. Done means the sync API no longer relies on deprecated depth while preserving correct topological ordering; the issue also notes that a better ordering approach may be needed.

Written by the indexing model from the issue text.