redis: Log spam warning when redis is loading the dataset in memory

Created by: ijsnow

Some concerning repo-updater logs were reported from a customer:

22:08:33       repo-updater | t=2019-03-06T22:08:33+0000 lvl=warn msg="failed to execute redis command" cmd=SETEX error="LOADING Redis is loading the dataset in memory"
22:08:33       repo-updater | t=2019-03-06T22:08:33+0000 lvl=warn msg="failed to execute redis command" cmd=SETEX error="LOADING Redis is loading the dataset in memory"
22:08:33       repo-updater | t=2019-03-06T22:08:33+0000 lvl=warn msg="failed to execute redis command" cmd=SETEX error="LOADING Redis is loading the dataset in memory"

These logs didn't appear to correlate with any broken functionality but we should be aware and investigate eventually. They were noticed while looking into the cause of another issue and ultimately took up a non-zero amount of time that could have been used focusing on the issue at hand.