When user creates a new silence with regex match it's left to the user to anchor it, which is not consistent with Prometheus regex handling (promql/functions.go#L818).
Turn the GroupKey into a string that is composed of the matchers if the
path in the routing tree and the grouping labels.
Only hash it at the very end to ensure we don't exceed size limits of
integration APIs.
This commit removes the dependency on model.Silence for the internal
Silence type, uses UUIDs instead of uint64s and clarifies invariants
around timestamp handling.
The created_at timestamp is removed for the time being.
