Whether the header was set by the backend or directly in VCL, it is now
possible to signal that a backend connection should not be added back to
the pool after a successful fetch with a Connection:close header.

Pooling such a connection would be counter-productive if closing the
session was requested by the backend itself, because it would then be
likely that reusing the connection would result in busting the extra
chance.

Setting the Connection:close directly in VCL can help mitigating against
a misbehaving backend.

Refs #3400