Drop content_sao.{cpp,h}

Move LuaEntitySAO to a new dedicated file
Drop TestSAO (useless object)
Drop the old static startup initialized SAO factory, which was pretty useless.
This factory was using a std::map for 2 elements, now just use a simple condition owned by ServerEnvironment, which will be lightweight, that will also drop a one time useful test on each LuaEntitySAO creation. This should reduce server load on massive SAO creation

src/CMakeLists.txt

@@ -375,7 +375,6 @@ set(common_SRCS
 	collision.cpp
 	content_mapnode.cpp
 	content_nodemeta.cpp
-	content_sao.cpp
 	convert_json.cpp
 	craftdef.cpp
 	debug.cpp