Local discovery: Use db instead of filesystem

We mostly trust the file watchers meaning that we don't re-scan the
local tree if we have done that recently and no file watcher events
have arrived. If the file watchers invalidate a subtree, we rescan
only that subtree.

Since we're not entirely sure the file watchers are reliable, we still
do full local discoveries regularly (1h by default). There is a config
file setting as well as an environment variable to control the interval.

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