Fix max frame size

[mattsb42-aws]

Problem

Per the message format specification, the frame size is an unsigned 32-bit integer. No other limits are set on this value. As such, the max frame size is 2^32 - 1, not 2^31 - 1 as currently defined. It looks like this snuck in as an implementation detail from the Java client, where the frame size is limited by MAX_INT.

Solution

Fix the defined value to be 2^32 - 1 and add tests that verify that frames of size 2^31 - 1 < frame_size <= 2^32 - 1 work correctly.