Fix max frame size

## 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.