With this change in place, the grafana service will signal
readiness to serve by writing "READY=1" to the path specified
through the NOTIFY_SOCKET environment variable. If this
environment variable is not present or empty, no notification
will happen. This notification is the standard systemd
mechanism for indicating a service is ready to serve. For
Grafana this may be a couple of seconds from startup due to
database migrations. This change also adjusts the Grafana
systemd service definition to make use of this feature.
In systemd `After=` merely codes an ordering relation and does not start
other units.
Grafana needs to specify that it wants to start up _after_ either of the
supported DB services have started, _if they start_.
Enabling the database services, however, is out of scope for Grafana.
