A synckey that is not matching the server sync key is invalid.

Summary:
The problem I've been running into:

• The server sent an update to the client that the client rejected (due

to "invalid folders"

• The client sends a sync request with an "old" synckey
• The server should now tell the client that the synckey is invalid,

but instaed silently accepts the synckey reporting no problem.

• The client now has a different folder set than the server, which

causes problems in Sync and Ping requests.

Differential Revision: https://git.kolab.org/D2128