Use HEAD request to get digest

[bwateratmsft]

This change does several things:

1. Uses the Accept: application/vnd.docker.distribution.manifest.list.v2+json header to enable doing HEAD requests--not subject to rate limiting--to check if an image is out-of-date
2. Re-enables out-of-date checking by default
3. Returns out-of-date checking to once per session instead of once per day. I don't feel strongly that it ought to be once-per-session, so if desired I can keep it at once-per-day.

I'd be OK with putting this in either 1.10.0 or 1.11.0.

[BigMorty]

This is great news!!! I am OK with once a day also.

[bwateratmsft]

This is now released in Docker extension version 1.10.0.