server: Extract most logic into the new socket abstraction

- Move event loop code into event/socket
- Reimplement server.c on top of the new SocketWatcher class
- Adapt msgpack_rpc/channel.c

src/nvim/msgpack_rpc/channel.h

@@ -5,6 +5,7 @@
 #include <uv.h>
 
 #include "nvim/api/private/defs.h"
+#include "nvim/event/socket.h"
 #include "nvim/vim.h"
 
 #define METHOD_MAXLEN 512