The field is still optional, but if no default key is specified, then
it is implicitly the empty string. We no longer support a use case
in which an error response is returned for sharding by key if the
key is missing from the request. This will also be the case when
we support sharding by other keys besides cookies, such as request
headers.

This makes it possible to move the clause in vcl_recv that includes
the synthetic response to health checks from other shard instances
to come after the code for primaryOnly. That had been moved to the
start of vcl_recv, because health checks were failing in the shard
by cookie case without a default key (since health probes have no
Cookie header). But the result was that primaryOnly code did not
work as expected. Now we have moved that clause back to where it
was previously -- health probes now don't fail in the shard by
cookie case, because there is always a default key.