Fix the value of `Gp_max_tuple_chunk_size`

[SmartKeyerror]

This PR is trying to fix the problem that there are 4 bytes in IC package that are never used.

When we set gp_max_packet_size to the minimum value of 512 and put a breakpoint in QD's RecvTupleChunk function, we can find that the value of conn->msgSize is only 508, and the correct value should be 512. This is because when calculating the longest tuple length, an additional TUPLE_CHUNK_HEADER_SIZE is subtracted, which is 4 bytes:

Thread 1 "postgres" hit Breakpoint 1, RecvTupleChunk () at ic_common.c:93
(gdb) n
(gdb) p conn->msgSize
$10 = 508
(gdb) n
(gdb) p tcSize
$11 = 444