lfs: debug print HTTP headers and JSON payload received from the server
This has been extremely valuable to show divergences between hg serve and
lfs-test-server. Once the hg serve code lands, there will be a certain
amount of conditionalizing that needs to be done, because lfs-test-server
doesn't always follow its spec.
The $ISO_8601_DATE_TIME$ pattern masks the fact that lfs-test-serve is sending
out an expires_at value of "0001-01-01T00:00:00Z". hg serve will (probably)
use current time + 10 minutes or similar. The $HTTP_DATE$ is the current time.