Plot protection & terrain generation

# Changelog:

#### Plot protection
* Extending piston can no longer move plot border
* Retracting sticky piston can no longer move plot border

#### Terrain generation
* Added stairs on road borders
* Changed plot border to double stone slabs


# Known issues:

#### Plot protection
* Beds can be placed on plot border (bed takes 2 blocks, if you place it near plot border, second block will appear on the border)
* Blocks can be placed above border (when moved with piston)

#### Claiming plots
* Plot owner is saved as null if player disconnects before server restarts (possible fix: save right after plot is claimed?)

#### Terrain generation
* Plot signs for some plots are facing wrong direction