Add tcp_freecb() - single place to free tcpcb.

[glebius]

Summary

Until this change there were two places where we would free tcpcb -
tcp_discardcb() in case if all timers are drained and tcp_timer_discard()
otherwise. They were pretty much copy-n-paste, except that in the
default case we would run tcp_hc_update(). Merge this into single
function tcp_freecb() and move new short version of tcp_timer_discard()
to tcp_timer.c and make it static.

[• hselasky]

Looks good.

[glebius]

Committed ff94500855c16d0d9cc18aa8b0ba73ea94020c56