tcp: adding a functionality to define "trace points" so that BB logging can be enabled at specific events.

This commit will add a new concept to rack, tracepoints. A tracepoint
is a defined point inserted into the code (3 are included in this initial patch) that
allows a developer to insert a point that might be of interest. The developer numbers
the point in the tcp_rack.h file and then can use sysctl to enable that (or all) trace
points. A limit is also given to how many BB logged connections will turn on
so that a box is not overrun by BB logging.

Reviewed by: tuexen
Sponsored by: Netflix Inc.
Differential Revision: https://reviews.freebsd.org/D34898