get_many sometimes fails to bring back details cached on one or more Memcached nodes. It doesn't look like these keys are being evicted. 99% of the time all keys come back, but 1% of the time all the keys stored on one or more nodes are missing from the result of the get_many. One get_many may fail to bring these keys back, but a get_many occurring immediately afterwards will succeed in bringing back all the keys. This behavior can be mitigated by trying again for the missing keys, which is what we do here. Splitting the get_many into smaller chunks may also help.
LEARNER-382
