Share `Octopus::Proxy` between threads

Only the accessors must be scoped to the thread, the rest of state can
be shared.
Use custom postgres user when available on advert model
Fixing spec suite
`Module.class_variable_set` is private on ruby 1.8
Use accessors instead of `Thread.current` on `Octopus::Proxy#clean_proxy`
Merge pull request #163 from tchandy/share-octopus-proxy-between-threads

Share octopus proxy between threads