waiverdb: Set fixed number of processes ans threads
    
    New waiverdb container image starts 8 gunicorn web worker processes by
    default using WEB_CONCURRENCY environment variable. This causes memory
    to spike (over 500MiB) and workers to get terminated.
    
    Instead of increasing memory limits, a better solution is to decrease
    the number of processes and increase number of threads since the app
    mostly waits on DB requests to finish and waiverdb workers themselves
    are thread-safe.