Local variables can overflow when serializing a 2GB message

When rpc_max_message_size is set to its maximum of INT_MAX (2147483647), certain local variables in SerializeMessage can overflow as messages approach this size. Specifically, recorded_size, size_with_delim, and total_size are 4 byte signed integers and could overflow when additional_size becomes large.

Since INT_MAX is the largest allowable value for rpc_max_message_size (a 4 byte signed integer), these variables will not overflow if changed to 4 byte unsigned integers. This would eliminate the potential problem for serialization.

A similar problem exists in the InboundTransfer::ReceiveBuffer() and similar codepaths. Changing those variables to unsigned integers should resolve the issue.

This does not impact existing systems, because the default value of rpc_max_message_size is 50MB.