Measure sync performance on each platform

We believe we have widespread performance issues associated with sync, motivating the integration of sliding sync.

We should measure:

* Time taken for an initial sync after app start / login
* Time taken for an incremental sync, including as a property the time since the last sync (so we can quantify how sync delay varies according to time since last update)

Start by identifying the sync entry points and platform-specific client implementations, since the issue names no files or tests. Define how initial sync and incremental sync durations will be measured, including time since the previous sync, and consider the work complete when those measurements are available for each platform.

Written by the indexing model from the issue text.