Switch shared_ptr pimpls to unique_ptr.

This also makes objects which "own" an actor non-copyable, which I think
makes sense.  E.g. a master data store clearly shouldn't have a copy.
Copies may also be tricky to define, e.g. any data store frontend may
have outstanding queries when a copy is made -- should the copy also
receive a response?  To truly be a copy, maybe it should, but that's
also not intuitive.  Currently, I find allowing copies not useful enough
to warrant complications they'd impose.