Ignore notfound folders.

Summary:
This is a workaround for Outlook continuing to request the same removed
folders over and over.
By instead simply ignoring the request we avoid the endless loop and
restore normal functionality otherwise.

The downsides are:

• A Sync command will no longer detect missing folders and thus trigger a FolderSync
• The client is left in a state where it expects a folder to be

synchronized, while it really isn't.

Differential Revision: https://git.kolab.org/D2074