memcached logs flooded with "Failed to fetch data from database"

Found whilst debugging MB-20549, the memcached log for 172.23.105.83 only contained 4 hours of history, completely masking the root cause. Of the 1.2M lines in the log file, 1.1M of them were along the lines of:

2016-08-15T08:11:09.355210-07:00 WARNING (default) Failed to fetch data from database, vBucket=651 error=document not found [none]

i.e. the log generated here

If this error is capable of occurring so profusely, we should find a way to suppress its repetition, or perhaps convert it into a stat.

Original offending log file is here:
https://s3.amazonaws.com/scalability-mcafee/MB-19187-4.5.1/collectinfo-2016-08-15T190253-ns_1%40172.23.105.83.zip