Spatialite topology is super slow

[davenquinn]

In Spatialite mode, reasonably complex topologies take tens of seconds to refresh. Because Spatialite does not support concurrent writes, this blocks other interactions with the database (e.g., drawing) until the update is done.

There are several things that might contribute to a fix:

• Work more iteratively. Right now, we basically bulk-recreate topological tables; we might be able to proceed feature-by-feature more effectively. This would require a better understanding of the TopoGeo_FromGeoTableExt function.
• Split single database transaction or use "concurrent reads" so more can happen while the topology update transaction is open
• Update GRDB to the latest version

We could even do all actual topology work on a separate database, but that would likely be overkill...

[davenquinn]

This might be something we can fix alongside #1...