tcp: hpts needs to still call output even after input.

The other stacks it turns out actually expect the output to be called and can become stuck if it is
not. This is because they run there timer code from there and the input routine does not always
assure a timer is running. The real longterm fix here might be to go into the other stacks (rack and bbr)
and make sure that a timer is running after input if you don't do output.. as well as call the timer functions.
This would cut down on calls from hpts. But I think its too dramatic of a change for the immediate time.

Reviewed by: tuexen, glebius
Sponsored by: Netflix Inc
Differential Revision:https://reviews.freebsd.org/D39738