Fix USA Sentry Drone attack issues

[xezon]

When ordering one or more Sentry Drones to attack an enemy unit, they often just don't attack at all. Or move straight to the enemy unit without firing at the first possible chance.

[commy2]

The Sentry Drone has an invisible deploy mode similar to the Nuke Launcher. It takes 1 second of standing still to transition between movement mode and attack mode, and by clicking fast, you can get stuck deploying and undeploying in a loop.

One way to fix this is to reduce the deploy time (e.g. 500 msec) which from messing around with it feels much better.
See commy2/zerohour@a67c0d3

Another option is to remove the packing time completely.

[commy2]

Closing as duplicate of #862. The newer issue contains more information.