Pet Count

If we have enought pets exist, do not call new pets again.
Because I am only familiar two characters, currently support skills:
DemonHunter_Sentry, Witchdoctor_Gargantuan, Witchdoctor_SummonZombieDog,
Witchdoctor_FetishArmy.

If two skills are using at the same time, one may be missed but its
timer was re-initailled. Big delay will cause the missed skill to be
used again long time later.
To solve that, I change the delay to small value. Using skill is
determind by IsPowerReady() in launch_spell(), so we are not afraid that
Skill Key is pressed too offent.