When the reconstructor fails to gather enough fragments to rebuild a
missing fragment, log more details about the responses that it *did*
get:
- log total number of ok responses, as well as the number of useful
responses, to reveal if, for example, there might have been
duplicate frag indexes or mixed etags.
- log the mix of error status codes received to reveal if, for
example, they were all 404s.
Also refactor reconstruct_fa to track all state related to a timestamp
in a small data encapsulation class rather than in multiple dicts.
Related-Bug: 1655608
Change-Id: I3f87933f788685775ce59f3724f17d5db948d502
7960097f02