[cnc] Airstrike cooldown 3 mins. (from 4). tweaks.

Air Strike cooldown time reduced from 4 minutes to 3. (Ion Cannon is 3). [games are short in CNC, and waiting 4 minutes for your first airstrike is a long time.]
HQ's hitpoints reduced to 750, so it can be nuked in one shot. (but not Ion Cannon'd) 
Airfield HP reduced to 1500, so Ion + Airstrike can destroy it. (For some reason, it doesn't die as easily as Weapons Factory, despite having the same HP)