RFC: use unix domain socket for internal RPC of gateway

[mitake]

The root cause of the recent problem fixed in #10366 is that etcd uses a certificate to itself. If etcd uses non TLS unix domain socket for the internal RPCs from gateway, the possibility of such a problem can be much more reduced.

cc @hexfusion @gyuho @xiang90 @jpbetz

(socketpair(2) is the ideal solution because it is completely invisible from outside but grpc-go seems to be not supporting creating connection from fd: grpc/grpc#6610)

[stale]

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