refactor(msgpack): allow flushing buffer while packing msgpack

Before, we needed to always pack an entire msgpack_rpc Object to
a continous memory buffer before sending it out to a channel.
But this is generally wasteful. it is better to just flush
whatever is in the buffer and then continue packing to a new buffer.

This is also done for the UI event packer where there are some extra logic
to "finish" of an existing batch of nevents/ncalls. This doesn't really
stop us from flushing the buffer, just that we need to update the state
machine accordingly so the next call to prepare_call() always will
start with a new event (even though the buffer might contain overflow
data from a large event).

src/nvim/generators/gen_api_dispatch.lua

@@ -307,7 +307,6 @@ output:write([[
 #include "nvim/globals.h"
 #include "nvim/log.h"
 #include "nvim/map_defs.h"
-#include "nvim/msgpack_rpc/helpers.h"
 
 #include "nvim/api/autocmd.h"
 #include "nvim/api/buffer.h"