core: make fixed latency dynamically changeable

core: make fixed latency dynamically changeable

This of course makes the name 'fixed' a bit of a misnomer. However the
definitions are now like this:

fixed latency: the latency may change during runtime, but is solely
controlled by the backend, the client has no influence.

dynamic latency: the latency may change during runtime, influenced by
the requests of the clients.

i.e. fixed vs. dynamic is from the perspective of the client.