We promote 'struct json_stream' to contain the membuf; we only attach
the json_stream to the command when we actually call
json_stream_success / json_stream_fail.
This means we are closer to 'struct json_stream' being an independent
layer; the tests are already modified to use it directly to create
JSON.
This is also the first step toward re-enabling non-serial command
execution.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
