Optimize HPTS so that little work is done until we have a hpts thread that is over the connection threshold

HPTS inserts a softclock for system call return that optimizes performance. However when
no HPTS threads need the help (i.e. when they have less than 100 or so connections) then
there should be little work done i.e. check the counter and return instead of running through
all the threads getting locks etc.ptimize HPTS so that little work is done until we have a hpts
thread that is over the connection threshold.

Reported by: eduardo
Reviewed by: gallatin, glebius, tuexen
Tested by: gallatin
Differential Revision: https://reviews.freebsd.org/D44420

(cherry picked from commit b7b78c1c169dd2213b4cb3e14e19c045b2c5e5af)