A service for read-only mode

A service for read-only mode

Introduce a service to represent wfReadOnly() and friends.

It's necessary to have two service instances, one for wfReadOnly() and
one for wfConfiguredReadOnlyReason(), to avoid a circular dependency,
since LoadBalancer needs the configured reason during construction, but
wfReadOnly() needs to query the currently active load balancer.

Not having a cache of the configuration makes it possible to dynamically
change the configuration. Ideally things would not change the
configuration, and I removed such instances in core, but to support
extensions, I added a test ensuring that the configuration can be changed.

Change-Id: I9bbee946c10742526d3423208efd68cb3cc5a7ee