Now that we have json_stream in common/, we can move all the related
helpers from lightningd/json to common/json. This way everyone can
benefit of them (including libplugin, the plugins themselves,
potentially lightning-cli), not lightningd alone!
Note that the Makefile of the common/test/ had to be modified, because
the new helpers make use of common/wireaddr... Which turns out to
\#include <lightingd/lightningd.h> ! So we couldnt just include the .c
and add mocks if we redefined some structs (hello run-param).
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