Fix resolved alerts still inhibiting

[stuartnelson3]

Update old alert with result of merge with new

On ingest, alerts with matching fingerprints are
merged if the new alert's start and end times
overlap with the old alert's.

The merge creates a new alert, which is then
updated in the internal alert store.

The original alert is not updated (because merge
creates a copy), so it is never marked as resolved
in the inhibitor's reference to it.

The code within the inhibitor relies on skipping
over resolved alerts, but because the old alert is
never updated it is never marked as resolved. Thus
it continues to inhibit other alerts until it is
cleaned up by the internal GC.

This commit updates the struct of the old alert
with the result of the merge with the new alert.

An alternative would be to always update the
inhibitor's internal cache of alerts regardless of
an alert's resolve status.

This fixes #1153. Thanks to @simonpasquier for doing the bulk of the work :)

EDIT:
This PR has been modified to always update the inhibitors internal cache, whether the incoming alert is resolved or not.

[stuartnelson3]

As mentioned in the description, this line could be removed if the inhibitor's internal scache is updated even if an alert is resolved.

I think I would prefer that, as opposed to updating a pointer's value and hoping that it's being referred to elsewhere.

[stuartnelson3]

Had a chat with @grobie and he agreed. The latest commit reflects these changes.

[grobie]

LGTM! Awesome test coverage!

Looks like a very small optimization we lose here.

[simonpasquier]

👍