Currently, the event-list output is very resource-centric, despite
being scoped to the stack from an API path perspective.
This, combined with the fact that the stack updated_at timestamp
is only updated after a succesful update (ref bug #1193269) makes
it impossible to derive the time when an update started via any API.
This is a problem when trying to use the new hook/breakpoint API,
because it's necessary to poll for all events since the most recent
update started, disregarding any stale hook events from previous
updates (which may have failed or timed out without the hooks getting
cleared).
To work around this, add an event for each stack state transition,
such that you can detect the transition to UPDATE_IN_PROGRESS,
then use that event as a marker to get post-update-started events.
Without this (or some other way to determine when the stack update
started), the hooks pre-update functionality landed for kilo is
not really usable (particularly mechanically via scripts).
Change-Id: Idff342b3aecc2d145dfbc7c0f610ad0ca8e52c8b
Partial-Bug: #1448155
d5793c08e0