TCP: Misc cleanups of tcp_subr.c

In going through all the TCP stacks I have found we have a few little bugs and niggles that need to
be cleaned up in tcp_subr.c including the following:

a) Set tcp_restoral_thresh to 450 (45%) not 550. This is a better proven value in my testing.
b) Lets track when we try to do pacing but fail via a counter for connections that do pace.
c) If a switch away from the default stack occurs and it fails we need to make sure the time

scale is in the right mode (just in case the other stack changed it but then failed).

d) Use the TP_RXTCUR() macro when starting the TT_REXMT timer.
e) When we end a default flow lets log that in BBlogs as well as cleanup any t_acktime (disable).
f) When we respond with a RST lets make sure to update the log_end_status properly.
g) When starting a new pcb lets assure that all LRO features are off.
h) When discarding a connection lets make sure that any t_in_pkt's that might be there are freed properly.

Reviewed by: tuexen
Sponsored by: Netflix Inc
Differential Revision:https://reviews.freebsd.org/D39501