To ensure that health checks from other Varnish instances succeed
in the self-sharding configuration, move the clause in vcl_recv
that is executed when remote.ip matches the ACL for cluster members
to the start of the subroutine.

XXX: currently returns a synthetic 400 response if the configured
cookie name is not found in the Cookie header (or if there is no
Cookie header). In a further iteration, we will add an optional
field defaultKey, which if present sets a string value to be used
as the sharding key if the key is not found.

XXX: testing this feature has revealed bugs in sharding by key
(using by=KEY) unless primaryOnly is also set to true. These will
be fixed in a further iteration.