Signed-off-by: beorn7 <[email protected]>

Add version tracking of silences states. Adding a silence to the state
increments the version. If the version hasn't changed since the last
time an alert was checked for being silenced, we only have to verify
that the relevant silences are still active rather than checking the
alert against all silences.

Signed-off-by: beorn7 <[email protected]>

Signed-off-by: beorn7 <[email protected]>

Essentially, the Silences.Expire() will in that case have no effect
because the affected silence is immediately seen as expired from the
storage and thus not updated. The silence will stay around in its old
state.

This fix makes sure to use the same “now” throughout the expiration
process.

Signed-off-by: beorn7 <[email protected]>

Signed-off-by: beorn7 <[email protected]>

Without the `-u`, it will load what's required in the `go.mod`
file. But with the `-u`, it will load new stuff once it's available,
which makes the build non-reproducible. (Without any change in the
`errcheck` repo, other things will happen.)

Signed-off-by: beorn7 <[email protected]>