Allow 2 TempURL keys per account.

This allows users to rotate their TempURL keys without invalidating
all existing signed URLs. This is handy if you have multiple systems
generating signed URLs, but you want to change your keys for some
reason (e.g. keys compromised, company policy, general paranoia).

Both the first and second keys are optional, so existing accounts'
signed URLs will continue to work as before.

This commit does change the memcache key used to store the fetched
TempURL keys. This is because we were storing the old key as a string
in memcached, but the new one is a list of keys. Since the key cache
lifetime here is only 60 seconds, it doesn't seem like too big a deal
to completely flush the TempURL cache.

Also, this commit adds caching of a negative TempURL result. If the
account HEAD reveals no TempURL keys at all, that result is now stored
for 60 seconds the same way that a positive result would be.

Change-Id: I40a02bd607283fbce11aa52a9bb8a5846ab17f5e
Merge "Allow 2 TempURL keys per account."