Fix: old task trackers were not really removed

[arikfr]

Due to stupid mistake, old task trackers are not removed from Redis which results in ever increasing memory usage.

This pull request fixes this, but if you want to cleanup your instance, you can use the following script:

from redash import redis_connection

if __name__ == '__main__':
    for k in redis_connection.keys('query_task_tracker:*'):
        if redis_connection.zscore('query_task_trackers:done', k) is None and redis_connection.zscore('query_task_trackers:waiting', k) is None and redis_connection.zscore('query_task_trackers:in_progress', k) is None:
            redis_connection.delete(k)   

This script is not efficient, but will do the job. Note that it supposed to be run from your Redash directory (usually /opt/redash/current) using the bin/run utility.