update guitar link to include the new conf.Settings struct, and add it to GraphStoreRequest interface and implementors.

existing code works unchanged with the 'zero'-value default.
update tests: since all the test tar streams neglected to set modtime dates, they now (correctly) don't serialize them as well.
put some modtime values into the graph integration tests
add flag to build subcommand for forcing epoch timestamps, and pass this through to where graph can pick up and tell guitar about it.
Merge branch 'support_temporal_ignorance'
fix some issues loading images introduced with branch name refactor
more internal renames mysteriously missed in the last docket->hroot regex.