handle doc deletes in sql pillows

Looks like this allows at most one retry immediately after the failure. I couldn't find [much information](http://initd.org/psycopg/docs/module.html#psycopg2.InterfaceError) about `InterfaceError` so not sure of the conditions in which it is raised, but wondering if it would make sense to implement multiple retries with exponential backoff (retry in 2s, 20s, 200s, for example)? I suppose that would be a new feature, so could be implemented later.