Simplify Strongback engine and executor

[rhauch]

There are several things that need to be simplified/corrected:

• the way Strongback.java sets up the components is far too complex;
• the logic for running the separate thread needs to be simplified; and
• the way execution delays are computed is incorrect and should be simplified.

We should make these changes to the 1.x branch so teams still using Strongback with the 2016 WPILib can get these improvements. It still is a short bit before the 2017 WPILib will be released, but teams may still want to continue using 2016 WPILib for some time. Consequently, we'll probably include these as a 1.2.0 release (rather than a 1.1.8 patch release on the existing line), but will also definitely include these improvements in the Strongback for 2017.