Allow exclusive dependency on stream 0.

Right now we default `depends_on` to `None` because we want to distinguish "not set" from "0" for the purposes of handling things like exclusive dependency. This doesn't actually work: nothing in RFC 7540 forbids the setting of exclusive dependencies on stream 0. So we should simply adjust the codebase to set all `None` values of `depends_on` to `0` and then proceed from there.