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>