Prevent same instance from being selected twice in a row

Introduces a new db key "<service>-previous" to track which instance was
last selected for a particular service. This allows for filtering the
list of available instances to exclude the instance that was last
picked, to ensure a (slightly) more even distribution of traffic.
There's still the possiblity of the following scenario, however:

:service instances > 2

/:service request #1 -> instance #1
/:service request #2 -> instance #2
/:service request #3 -> instance #1
/:service request #4 -> instance #2

where there are many ignored instances for a particular service. One
possible solution would be to implement the "<service>-previous" value
to be a list, rather than a single value, and push to that list until
only one element is left in the original "instance" array after
filtering, and then delete the "<service>-previous" key.

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