Now that transitioned to responding to commit requests from host
rather than individual endpoints on the host, updated variable names
to reflect this (ie, change from endpoint_uuid to host_uuid in a bunch
of places).
Each endpoint has a single active event per running transaction now.
(In previous versions, making an endpoint call on another endpoint
would create a new event for that endpoint.)  This means that we can
get rid of a lot of the machinery for tracking multiple events on the
same host.
Explicitly pass in threadpools, host uuids, etc that each activeevvent uses rather than just chaining from endpoint.
Relying solely on each hosts map of all events, instead of keeping a
separate submap as well for each endpoint.