Default to 4K receive buffer size

Change the _sync method so that when no specific size is requested it
fetches up to 4096 bytes instead of 1 byte at a time from the
operating system. It's not clear what the reason was for setting it 1
byte. It was introduced in pymodbus commit:

9d2c864

Patch:

3b490fe

But there's no rationale there for setting it to 1 byte specifically.
While it may have been to handle some obscure operating system with a
poor network stack implementation this should have been documented, as
it is written it comes across as that the implementer didn't
understand how the TCP/stream socket networking API works.

Note that https://docs.python.org/3/library/socket.html does not
contain a full description on the socket.recv method; see also pydoc
for socket.socket.recv.

Author: thormick

pymodbus/client/sync.py

@@ -261,9 +261,9 @@ def _recv(self, size):
 
         timeout = self.timeout
 
-        # If size isn't specified read 1 byte at a time.
+        # If size isn't specified read up to 4096 bytes at a time.
         if size is None:
-            recv_size = 1
+            recv_size = 4096
         else:
             recv_size = size