Improved readiness probe, database services only use ready pods

[ewoutp]

This PR changes the readiness tests of some pods & tweaks the services used to access the database.

• The readiness probe is now added to all single & coordinator pods (before only coordinator)
• The readiness probe is done a lot quicker and more often
• The readiness probe is done on _admin/echo for activefailover deployments
• The database & external access services no only accept ready endpoints

As a result, activefailover deployments only expose the master single server.
Also a cluster deployment exposes only ready coordinators.

[neunhoef]

LGTM