tcp: The rack stack can incorrectly have an overflow when calculating a burst delay.

[rrs]

Summary

If the congestion window is very large the fact that we multiply it by 1000 (for microseconds) can
cause the uint32_t to overflow and we incorrectly calculate a very small divisor. This will then
cause the burst timer to be very large when it should be 0. Instead lets make the three variables
uint64_t and avoid the issue.

[rrs]

While where at it, there is a duplicate srtt that should be just one var at the higher scope. Lets fix
that and make the needed casts when comparing to slot. That way we never can get a
confused compiler :)