rack may end up with a stuck connectin fi the rwnd is colapsed on sent data.

There is a case where rack will get stuck when it has outstanding data and
the peer collapses the rwnd down to 0. This leaves the session hung if
the rwnd update is not received. You can test this with the packet drill script
below. Without this fix it will be stuck and hang. With it we retransmit everything.
This also fixes the mtu retransmit case so we don't go into recovery when
the mtu is changed to a smaller value.

Reviewed by: tuexen
Sponsored by: Netflix Inc.
Differential Revision: https://reviews.freebsd.org/D34573