Actually this renames git_commit_create_oid to
git_commit_create_from_oids and moves the API declaration to
include/git2/sys/commit.h since it is a dangerous API for general
use (because it doesn't check that the OID list items actually
refer to real objects).
This moves some of the odb_backend stuff that is related to the
internals of an odb_backend implementation into include/git2/sys.
Some of the stuff related to streaming I left in include/git2
because it seemed like it would be reasonably needed by a normal
user who wanted to stream objects into and out of the ODB.
Also, I added APIs for traversing the list of backends so that
some of the tests would not need to access ODB internals.