Refactor target selection to fix bugs and improve performance

[welwood08]

• Fixes incorrectly using "displayed HP" which can be higher than real HP due to easing effect.
• Fixes incorrectly choosing last spawner below %hp threshold when spawner with lower %hp exists.
• Fixes incorrectly choosing last creep below %hp threshold when creep with lower %hp exists.
• Fixes incorrectly choosing lane with highest (or equal highest) raining gold but no enemies when enemies (even bosses and treasures!) still alive in other lanes.
• Fixes failing to choose highest raining gold lane when a spawner is alive with 100%hp in the same position in every lane and no other enemies exist.
• Fixes failing to choose highest raining gold lane when a creep is alive with 100%hp in the same position in every lane and no other enemies exist.
• Improves function performance more than 60% on average, sometimes up to 97%, by removing all the tightly nested loops and all the looping over the same things several times.

[welwood08]

I believe this also fixes the need to buy 1 auto-cannon before lane switching works, but that may have come in with rebasing on newer code.