Propagate the preferred_replicas to db::filter_for_query() and consider
them when selecting the endpoints. The algoritm for selecting the
endpoints is as follows:
* Compute the intersection of the endpoint candidates and the
preferred endpoints.
* If this yields a set of endpoints that already satisfies the CL
requirements use this set.
* Otherwise select the remaining endpoints according to the
load-balancing strategy, just like before.
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