config: Add max_worker_threads
    
    Concurrent async add_lockspace calls are limited by the number of worker
    threads. Using larger number of worker threads shorten the time to add
    many locksapces in large setups.
    
    Previously this value could be modified only via the command line. Now
    it can be modified via the sanlock configuration file for easier
    deployment.
    
    Buglink: https://bugzilla.redhat.com/1902468
    Signed-off-by: Nir Soffer <nsoffer@redhat.com>