tcp: Inconsistent use of hpts_calling flag

Gleb has noticed there were some inconsistency's in the way the inp_hpts_calls flag was being used. One
such inconsistency results in a bug when we can't allocate enough sendmap entries to entertain a call to
rack_output().. basically a timer won't get started like it should. Also in cleaning this up I find that the
"no_output" side of input needs to be adjusted to make sure we don't try to re-pace too quickly outside
the hpts assurance of 250useconds.

Another thing here is we end up with duplicate calls to tcp_output() which we should not. If packets go
from hpts for processing the input side of tcp will call the output side of tcp on the last packet if it is needed.
This means that when that occurs a second call to tcp_output would be made that is not needed and if pacing
is going on may be harmful.

Lets fix all this and explicitly state the contract that hpts is making with transports that care about the
flag.

Reviewed by: tuexen, glebius
Sponsored by: Netflix Inc
Differential Revision:https://reviews.freebsd.org/D39653