issue #139: core: Side.write(): let the OS write as much as possible.

There is no penalty for just passing as much data to the OS as possible,
it is not copied, and for a non-blocking socket, the OS will just keep
buffer as much as it can and tell us how much that was.

Also avoids a rather pointless string slice.

mitogen/core.py

@@ -664,7 +664,7 @@ class Side(object):
         if self.fd is None:
             return None
 
-        written, disconnected = io_op(os.write, self.fd, s[:CHUNK_SIZE])
+        written, disconnected = io_op(os.write, self.fd, s)
         if disconnected:
             return None
         return written