Add `testExistsHooks`.
Restore channel limits for `NoStore`.

Fix off-by-one errors in `CanStore()`.
Remove `TestStore`. Add `make test-memcached`.
`NoStore`: Assume all device IDs exist.

Closes #118.
Merge pull request #123 from mozilla-services/bug/no-store-exists

`NoStore.Exists()`: Assume all device IDs exist