Remove dependency on backdrop

Backdrop will create data sets on it's side as they're written to now
the dependency on backdrop can be removed from stagecraft.

This avoids the cross app transaction bugs around creating and deleting
data sets.

One thing to note is that when data sets are removed from stagecraft
the corresponding mongo collection will not be removed. As a
consequence, if the same data set is created again then the original
data and capped status will still be there, unless the collection is
first removed.