Sync worker

[psrpinto]

This issue serves to track PRs that have been open or need to be open upstream concerning element-hq#1045.

• Sync worker kickoff #11: Sync worker: kickoff element-hq/hydrogen-web#1056
• Communication between main thread and sync worker #13
• Run sync in worker #14
• Broadcast session and room changes from sync worker (no encryption yet) #15

[psrpinto]

Closing since upstream contributions can now be tracked through the upstream-contribution-candidate label.