Fix batched _send URL logic

Only the final batch of a commit call should use the commit URL since
the transaction would be closed after the request.

Signed-off-by: Byron Ruth <b@devel.io>
Simplify provenance loader

Fix #91

Signed-off-by: Byron Ruth <b@devel.io>
Move provenance under graph package

Also rename provenance.change => set to be consistent with graph
model function names.

Signed-off-by: Byron Ruth <b@devel.io>