This commit adds syncing support to the `StorageSyncArea` class, via
the Golden Gate library.
It also changes the `BridgedEngine` trait: `initialize` and `finalize`
haven't been useful in practice, since that's managed by the storage
service, and the `LazyStore` takes care of setting up the storage
connection on first use. But, what we do need is a way to signal a
sync is starting, so that the engine can set up temp tables. That's
handled by the new `sync_started`.
Finally, this commit changes `BridgedEngine::set_uploaded` to take a
`sync15_traits::Guid` instead of a `String`.
Differential Revision: https://phabricator.services.mozilla.com/D73415
8caa2f7986