Make trees much harder

[Froozigiusz]

I know that there is a damage/health rebalance in the far future, but this instance of a balance problem is too big.

When I was playing as a zombie, I was ble to take down a every tree with just three hits. Now I know this is not a problem with damage output, because other map elements needed much more hits in order to be destroyed. It's just a flora issue.