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.
Initial testing has shown BoltDB in plain usage to be a bottleneck
at a few thousand alerts or more (especially JSON decoding.
This commit actually makes them purely memory as a temporary solution.
Depending on the environment the timestamp internal fields
are different to handle timezones.
Thus a reflect.DeepEqual comparison falsely evaluates to
false.
This commit manually compares timestamps via the time.Time.Equal
method to circumvent the issue.
This commit implements creation and setting of silences.
They are stored as JSON objects in BoltDB and addressed by a unique
ID assigned on initial creation.
