xds: Use a grpcsync.Event to represent close in EDS LB policy (#3895)
xds: Use a grpcsync.Event to represent close in EDS LB policy (#3895)


grpc/grpc-go,dfawley/grpc-go,dfawley/grpc-go,stanley-cheung/grpc-go,grpc/grpc-go,stanley-cheung/grpc-go,menghanl/grpc-go,menghanl/grpc-go