http-walker: use object_id instead of bare hash

We long ago switched most code to using object_id structs instead of
bare "unsigned char *" hashes. This gives us more type safety from the
compiler, and generally makes it easier to understand what we expect in
each parameter.

But the dumb-http code has lagged behind. And indeed, the whole "walker"
subsystem interface has the same problem, though http-walker is the only
user left.

So let's update the walker interface to pass object_id structs (which we
already have anyway at all call sites!), and likewise use those within
the http-walker methods that it calls.

This cleans up the dumb-http code a bit, but will also let us fix a few
more commonly used helper functions.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>