The mgmt graph depends on state tracking to eliminate redundant pokes.
With the Watch loop now able to produce events quickly, it should no
longer play a part in determining the vertex state. This simplifies the
resource API as well!
This adds rate limiting with the limit and burst meta parameters. The
limits apply to how often the Process check is called. As a result, it
might get called more often than there are Watch events due to possible
Poke/BackPoke events.
This system might need to get rethought in the future depending on its
usefulness.
This patch makes a number of changes in the engine surrounding the
resource API. In particular:
* Cleanup of send/read event.
* Cleanup of DoSend (now Event) in the Watch method.
* Events are now more consistently pointers.
* Exiting within Watch is now done in a single place.
* Multiple incoming events will be combined into a single action.
* Events in flight during an action are played back after CheckApply.
* Addition of Close method to API
This gets things ready for rate limiting and semaphore metaparams!
This allows a resource to use polling instead of the event based
mechanism. This isn't recommended, but it could be useful, and it was
certainly fun to code!
