Replace lunajson with lua-cjson

[kaklakariada]

Currently we use lunajson, but lua-cjson is already available in Exasol.

Building lua-cjson on mac fails currently:

% luarocks install lua-cjson                 
Installing https://luarocks.org/lua-cjson-2.1.0.6-1.src.rock

lua-cjson 2.1.0.6-1 depends on lua >= 5.1 (5.4-1 provided by VM)
env MACOSX_DEPLOYMENT_TARGET=10.8 gcc -O2 -fPIC -I/usr/local/opt/lua/include/lua5.4 -c lua_cjson.c -o lua_cjson.o
lua_cjson.c:743:19: error: implicit declaration of function 'lua_objlen' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
            len = lua_objlen(l, -1);
                  ^
1 error generated.

See mpx/lua-cjson#54 for a potential solution.

Root cause: mpx/lua-cjson#56

Workarounds: Install it with flag -DLUA_COMPAT_5_3

luarocks install lua-cjson --local "CFLAGS=-O3 -Wall -pedantic -DNDEBUG -DLUA_COMPAT_5_3"

or pin the version: "lua-cjson == 2.1.0" in rockspec.