libtracker-miner: Renamed internal define TRACKER_MAX_TIMEOUT_INTERVAL

Use _CRAWLER_ in there so we know where it comes from.
libtracker-miner: Improve documentation for tracker_miner_fs_{s|g}et_throttle()
tracker-miner-fs: On error, return correct defaults for config

This covers tracker_config_get_crawling_interval() and
_get_removable_days_threshold() which were returning just 0.