Order peers in pick_peer callbacks

The order of peers presented to a diameter_app(3) pick_peer callback has
previously not been documented, but there are use cases that are
simplified by an ordering. For example, consider preferring a direct
connection to a specified Destination-Host/Realm to any host in the
realm. The implementation previously treated this as a special case by
placing matching hosts at the head of the peers list, but the
documentation made no guarantees. Now present peers in match-order, so
that the desired sorting is the result of the following filter.

  {any, [{all, [host, realm]}, realm]}

The implementation is not backwards compatible in the sense that a realm
filter alone is no longer equivalent in this case. However, as stated,
the documentation never made any guarantees regarding the sorting.

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