Make SyncManager.pull optional.

[jpoehnelt]

All changes could be set with the onChange callback in registerRemoteChange. However, after a mutation, the pull is called and there isn't an obvious source to use for the items array.

[jpoehnelt]

A workaround is to return an empty {changes: {}}.

[maxnowack]

I'm not sure if this would work. I think in the current implementation it's necessary to do at least one pull to have some initial data. Maybe this also leads to the issue you encountered in #1623

edit: just saw your PR #1638. Maybe the implementation is on some places more flexible than I thought 😀

[jpoehnelt]

I'm effectively using this PR, #1638, in my app right now. An issue which may or may not be related is handling failures on push. I'm not sure if this is related though. An initial hack which I haven't fully tested is:

changes.removeMany({collectionName: options.name});  // Probably could be more selective