KERNEL: Replace the library NXJSON(LGPL) with json-parser(BSD)

[Winlin]

Replace NXJSON for LICENSE issue: https://github.com/ossrs/srs/wiki/LicenseMixing#nxjson

[Winlin]

BSD: https://github.com/udp/json-parser
MIT: https://github.com/DaveGamble/cJSON
MIT: https://github.com/akheron/jansson
GPLv2+MPL+BSD: https://github.com/ossrs/state-threads#license

[Winlin]

The jansson doesn't support single file.
cJSON is not sure to work with C++.
Seems json-parser is fine.

[Winlin]

Update the research/api-server/server.py, response an object:

{
    "code": 0,
    "data": null
}

SRS http hooks also support a single code:int response, which is SRS1 style.

[Winlin]

Replace NXJSON which use LGPL.

[Winlin]

Fixed