Inline nlohmann/json for the JSON parsing itself, and handle requests
with the same SCGI interface as XML-RPC.
Based off the work in https://github.com/jesec/rtorrent
We can't have two classes with the same name in the rpc namespace. It causes ODR and lto-type-mismatches when compiling rTorrent with LTO.
This pull request addresses the problem by renaming the `xmlrpc_error` error class to `xmlrpc_error_c` in the xmlrpc_c file.
By default, builds will still not have XMLRPC enabled at all and the
configure flag `--with-xmlrpc-tinyxml2` must be specified. If both
xmlrpc-c and tinyxml2 are specified, xmlrpc-c takes precedence.
Basic benchmarks indicate tinyxml2 is 2x faster for small
requests/responses, and that only increases as response sizes get
larger.
Rosen Penev