wireprotoserver: access headers through parsed request
Now that we can access headers via the parsed request object, let's
do that.
Since the new object uses bytes, hyphens, and is case-insensitive, a
bit of code around normalizing values has been removed. I think
the new code is much more intuitive because it more closely matches
what is going out over the wire.
Differential Revision: https://phab.mercurial-scm.org/D2743
http://example.com/no/anchor, branches: (None, [])
http://example.com/an/anchor, branches: ('foo', [])
http://example.com/no/anchor/branches, branches: (None, ['foo'])
http://example.com/an/anchor/branches, branches: ('bar', ['foo'])
http://example.com/an/anchor/branches-None, branches: ('foo', [])
http://example.com/, branches: (None, [])
http://example.com/, branches: (None, [])
http://example.com/, branches: ('foo', [])