R1:eb73cb68ff5b

Raise a setup warning when locked configuration has a configuration value stored in the database

Summary:
Ref T13249. See <https://discourse.phabricator-community.org/t/configuring-the-number-of-taskmaster-daemons/2394/>.

Today, when a configuration value is "locked", we prevent //writes// to the database. However, we still perform reads. When you upgrade, we generally don't want a bunch of your configuration to change by surprise.

Some day, I'd like to stop reading locked configuration from the…