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/strings.c

@@ -2173,7 +2173,7 @@ String arena_printf(Arena *arena, const char *fmt, ...)
   char *buf = NULL;
   if (arena) {
     if (!arena->cur_blk) {
-      alloc_block(arena);
+      arena_alloc_block(arena);
     }
 
     // happy case, we can fit the printed string in the rest of the current