MB-7363: don't allow stacking of same poor man's alerts

Part of poor man's alerts is alert "key" which is used to prevent
duplicate alerts from reappering too often. We can use that same key
in order for fresher alerts to overwrite older alerts for same key.

This commit also makes metadata overhead and commit failure alerts
have per-bucket keys. So that alerts related to different buckets are
treated as separate alerts.

Change-Id: Ib6146b0a53d2b96c7dd5adf0c9998f3d51855df6
Reviewed-on: http://review.couchbase.org/26802
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>