feat(worker): allow number of threads to be configured

This allows for more threads to be run. Care needs to be taken to ensure that database connection pool size is appropriate for this.

doc/config/yaml.yml

@@ -47,6 +47,8 @@ worker:
   default_health_server_port: 9090
   # The default bind address for the worker health server to listen on
   default_health_server_bind_address: 127.0.0.1
+  # The number of threads to execute within each worker
+  threads: 2
 
 main_db:
   # Hostname for the main MariaDB server