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.

lib/postal/config_schema.rb

@@ -130,6 +130,11 @@ module Postal
         description "The default bind address for the worker health server to listen on"
         default "127.0.0.1"
       end
+
+      integer :threads do
+        description "The number of threads to execute within each worker"
+        default 2
+      end
     end
 
     group :main_db do