Since we flush memcached when we do a server restart, the flurry of
get_updates requests that fly in afterwards are all cache misses for
getting the User/UserProfile objects, so Tornado ends up spending
around 70ms per get_updates request rather than the usual 1-2ms.
So this should substantially improve our Tornado performance around
server restarts.
(imported from commit 07b8126bdfd4ff14e4c3362f9eda1fe5fd571c5b)
Our previous code could in theory end up clearing the caches it had
just filled, if Tornado's cache filling work happened to be faster
than the memcached flush.
(imported from commit 48174aadad398fb7a7c917a1df765c1261b12a55)
Tim Abbott