Manage the number of concurrent goroutines used for event processing

Adds the ability to throttle the number of concurrent goroutines used for event processing, to avoid excessive CPU usage.  Number of concurrent goroutines is limited to 'max_processes' config setting (defaults to 500).  Pending event queue is sized at 3*max_processes, to handle temporary inflow spikes.  When pending event queue is full, blocks with a timeout of 'wait_for_process' ms (defaults to 5 ms).

Minor fixes also included:
- fixed naming for event_handlers in config
- added timeout setting for webhook handler.  Defaults to 60s