Fallback directories don't serve extrainfo descriptors

Guess not too surprising, but most relays do not fetch extrainfo descriptors and as a result don't provide them when used as a fallback directory. For example...

# use Doedel26
% curl http://178.254.20.134:80/tor/extra/fp/9695DFC35FFEB861329B9F1AB04C46397020CE31
<no results>

# use moria1
curl http://128.31.0.39:9131/tor/extra/fp/9695DFC35FFEB861329B9F1AB04C46397020CE31
extra-info moria1 9695DFC35FFEB861329B9F1AB04C46397020CE31
published 2016-03-06 10:40:50
write-history 2016-03-06 06:56:34 (14400 s) 25821065216,26561069056,27690179584,25304364032,21429001216,15790654464
read-history 2016-03-06 06:56:34 (14400 s) 3077893120,3154214912,3207265280,3199601664,3018620928,2618984448
... etc...

Couple options come to mind...

• Only pick fallbacks that serve extrainfo descriptors.
• Don't use fallbacks when 'DownloadExtraInfo 1' is set.

Of those I suspect we want to opt for the second since few relays probably mirror this information.