GH-338 allow garbage collection of non-daemon processes

[spyoungtech]

As described in #338 use of atexit.register in AsyncAHKProcess causes a reference to be held for the subprocess. In use cases that make heavy use of run_script or calls with blocking=False, this can lead to buildup of memory. The rate of growth can be dependent on the size of the value returned by the script and how often it is called.

This PR fixes this issue by avoiding the need for atexit.register for these two scenarios. atexit.register is still used for the long-running daemon process to ensure cleanup.

[coveralls]

coverage: 76.87% (+0.2%) from 76.659%
when pulling 62e5940 on gh-338
into 46bce5c on main.

[spyoungtech]

This might also resolve #332