[tcp] Check Maximum Sizes TCP Can Handle

[BrianZill]

TCP sequence numbers are 32-bit values, which limits the "distance" between sequence numbers that can be reasonably compared to be "greater than" or "less than" the other. Also, even with window scaling, a TCP's advertised receive window can't be greater than 1 GiB. We should review the code to ensure that there aren't any places where we could overflow these values.

[This Issue is loosely linked to Issue #104]