MINOR: lua: register and execute converters in LUA

MINOR: lua: register and execute converters in LUA

This patch permits to write LUA converters. Note that
all the converters declared trough LUA are automatically
prefixed by "lua.".

The Lua converters needs the current session to be executed,
but the writed and executed Lua code must be static and
contextless. The TXN object is not created for the converters.