ensure scheduler runs for adding 1000 sources

[sssoleileraaa]

Description

Once #944 is merged, we will be using a scheduler to add source widgets to the client, only when no source widgets exist. There are couple reasons there may be source widgets already in the list when the client receives a large number of sources from the server in a sync:

1. The client was running when the server database was updated to insert a large number of sources
2. The client starts when there are already sources in the local database so the first run of the client adds source widgets for those sources, then the database syncs with the server database which has lets say 1000 sources and therefore the scheduler code from Retry fixing blocked UI on client start. #944 is not run.

Also we need to benchmark these changes to make sure that using the scheduler improves performance in these scenarios.

[eloquence]

Deferring to near-term backlog as it's not been a significant issue for pilot installs so far (from our discussion, it seems most likely to arise in scenarios when a very large sync occurs after a long break in previous usage). We'll revisit this in the next sprint (after the current 4/22-5/6 sprint), at the earliest.