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

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.

Reviewed by: Michael Tuexen
Sponsored by: Netflix Inc.
Differential Revision: https://reviews.freebsd.org/D32668