db-based S3 cache (#1440)

Working but since each request is a DB hit, unclear if it will ever be
that much faster.  Will try local django cache instead.
django local fs cache for file and video locations (#1440)
fix typo in default (unused) cache defn (#1440)
strip querystrings from file links (#1440)
fix comment: 4 hours, not 40 :) (#1440)
fix typo in removing_querystrings; remove from default case too (#1440)
fix: return values from dl_links_all had swapped params (#1440)

Caught by @jbau in code review.
cachestats to django log instead of apache log (#1440)

Caught by @jbau in code review
Merge pull request #1880 from Stanford-Online/sef-1440-s3-caching

Filesystem caching of S3 locations for files and videos (#1440)