remove NodeRegistry and LocalNode

LocalNode was only a collection of local dispatch methods that are
tightly coupled to Router. Moving them to Router allows LocalNode and
NodeRegistry to be removed.
add actor references

References are handles to remote actors. Avoids the need to deal with
routers and nodes directly.

For now references are name-based only, meaning the referenced actor
must be named on the remote node. Eventually mailbox references will be
supported also.