Support periodic refreshes in Adaptive mode

[jhmartin]

Looking at https://github.com/lettuce-io/lettuce-core/issues/339#issuecomment-241638972 it appears that there is a strong case to enable a periodic refresh even when using adaptive-refreshes. For example, a failed replica does not trigger a refresh -- clients first receive a trigger (such as a MOVED result) to trigger that kind of refresh, which won't necessarily happen.

Existing behavior could be achieved by making the 'default' interval exceptionally long and allowing the client to select a lower value.

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