Add a deleted_at to user_collections table

[pjenvey]

See rfk's comments here:

#22 (comment)

I believe the go version gets away without this field because it stores the top level modified timestamp separately in its per user KeyValues store.
mozilla-services/server-syncstorage#62

[jrconlin]

From what I've been able to see, the bug is that on a collections deletion:

1. the collection X-Last-Modified should be set to 0
2. the collection dropped from the /info/collections
3. Maintain the last time that the store was modified.

No. 3 seems like the trickier bit, and probably requires a "meta data" table.