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/shada.c

@@ -41,7 +41,6 @@
 #include "nvim/mbyte.h"
 #include "nvim/memory.h"
 #include "nvim/message.h"
-#include "nvim/msgpack_rpc/helpers.h"
 #include "nvim/normal_defs.h"
 #include "nvim/ops.h"
 #include "nvim/option.h"