Fix check for whether /tracker/data/json needs to be recomputed
In commit 49e287d2 "tracker_service: send a Last-Modified header for /tracker/data/json" I made the json_timestamp update conditional on whether the data had changed. That meant that after a few minutes, we would consider the data to be always stale, and recompute it every single time, as long as it didn't change. To fix this, use separate timestamps for "when did we check last" (json_timestamp) and "when did it change last" (json_last_modified).
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