Make TempURL more cache-efficient.

So, there's two cases here. In one case, we have memcache enabled;
account info gets cached in the usual place, and tempurl keys get
cached separately. In the other case, we have no cache. In neither
case is anything gained by having TempURL cache keys separately from
the account info.

This commit removes TempURL's custom caching logic and makes it rely
on get_account_info() instead.

Benefits include:
 * immediate visibility of new keys on account POST
 * less data in the cache per account --> more stuff fits in cache
 * less code for bugs to hide in

Change-Id: Idb0b6c165da14196b4c79149c546f0159b54edcb
Merge "Make TempURL more cache-efficient."