If you've got thousands of requests per second for objects in a single
container, you basically NEVER want that container's info to ever fall
out of memcache. If it *does*, all those clients are almost certainly
going to overload the container.
Avoid this by allowing some small fraction of requests to bypass and
refresh the cache, pushing out the TTL as long as there continue to be
requests to the container. The likelihood of skipping the cache is
configurable, similar to what we did for shard range sets.
Change-Id: If9249a42b30e2a2e7c4b0b91f947f24bf891b86f
Closes-Bug: #1883324
5c6407bf59