* Use Unix sockets for gRPC between co-located services in mini mode
In `weed mini`, all services run in one process. Previously, inter-service
gRPC traffic (volume↔master, filer↔master, S3↔filer, worker↔admin, etc.)
went through TCP loopback. This adds a gRPC Unix socket registry in the pb
package: mini mode registers a socket path per gRPC port at startup, each
gRPC server additionally listens on its socket, and GrpcDial transparently
routes to the socket via WithContextDialer when a match is found.
Standalone commands (weed master, weed filer, etc.) are unaffected since
no sockets are registered. TCP listeners are kept for external clients.
* Handle Serve error and clean up socket file in ServeGrpcOnLocalSocket
Log non-expected errors from grpcServer.Serve (ignoring
grpc.ErrServerStopped) and always remove the Unix socket file
when Serve returns, ensuring cleanup on Stop/GracefulStop.
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