tcp: Add a socket option to rack so we can test various changes to the slop value in timers.

Timer_slop, in TCP, has been 200ms for a long time. This value dates back
a long time when delayed ack timers were longer and links were slower. A
200ms timer slop allows 1 MSS to be sent over a 60kbps link. Its possible that
lowering this value to something more in line with todays delayed ack values (40ms)
might improve TCP. This bit of code makes it so rack can, via a socket option,
adjust the timer slop.

Reviewed by: mtuexen
Sponsered by: Netflix Inc
Differential Revision: https://reviews.freebsd.org/D30249

(cherry picked from commit 4f3addd94be5e02e6e425f6119f5409972ab5d14)