Bonus consistency: 416 responses now always have a body. Before, if
you had "swob.HTTPRequestedRangeNotSatisfiable()", you'd get a body,
but if you had "swob.Response(..., conditional_response=True)", then
you'd get a length-0 response body. Now you always get a response
body. It's just the default <html><h1>..., but at it's always there.
Bonus efficiency: do a little caching of sub-SLO manifests to avoid
needless re-fetches. This kicks in when there are multiple references
to the same sub-SLO in a given manifest. The caching only holds 20
sub-SLOs so that a malicious user can't build a giant SLO tree and use
it to run the proxy out of memory (we're already holding up to 10
manifests in memory at a time since a SLO can include another SLO to a
depth of 10; this doesn't make the situation too much worse).
Change-Id: I24716e3271cf3370642e3755447e717fd7d9957c
4bcd3d7f6d