Add --gen json:merge option, and disable merging by default
JSON generator: output complete descriptors
Merge branch 'json_fixes' into ZEDGE

Conflicts:
	compiler/cpp/src/generate/t_json_generator.cc
JSON generator: output complete descriptors
Merge branch 'json_fixes' into ZEDGE

Conflicts:
	compiler/cpp/src/generate/t_json_generator.cc