store: implement lowerencode in C

hgweb: run with "served" filter This changeset enable the "served" filter on all repo used by hgweb. Hgweb misbehave in a lot of when filtering changeset are present but I do not expect normal people to have secret or obsolete changeset on they server. Misbehavior will be gradually fixed later.

hgweb: walk the graph through the changelog This is necessary to enforce filtering. The result is a bit buggy (may provide less changeset than expected, but it will stop crashing on filtered revision access. Note that changelog.revs can not represents empty iteration like xrange did. So we have to explicitly prevent call when there is nothing to do.

hgweb: use changelog for iteration Iterating through changelog is necessary to enforce filtering.