Add valid_send_options/1 (#12)

Add an optional :offload send mode (#10)

This optional mode offloads the send/2 calls to the remote Manifold nodes
to one of the local `Manifold.Sender` processes.

Add a new test for the :offload send mode.

speed increases + support for sharded partitioner

1.3.0 includes performance improvements in #3, and the ability to shard partitioners in #4.