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/memory.h

@@ -45,6 +45,8 @@ EXTERN size_t arena_alloc_count INIT( = 0);
   ((v).capacity = (s), \
    (v).items = (void *)arena_alloc(a, sizeof((v).items[0]) * (v).capacity, true))
 
+#define ARENA_BLOCK_SIZE 4096
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "memory.h.generated.h"
 #endif