Since all API functions now run immediately after a msgpack-rpc request is
parsed by libuv callbacks, a mechanism was added to override this behavior and
allow certain functions to run in Nvim main loop.
The mechanism is simple: Any API function tagged with the FUNC_ATTR_DEFERRED (a
"dummy" attribute only used by msgpack-gen.lua) will be called when Nvim main
loop receives a K_EVENT key.
To implement this mechanism it was necessary some restructuration on the
msgpack-rpc modules, especially in the msgpack-gen.lua script.
