finite liquid

all liquids with constant volume and/or intresting effects like caves flood, bubbles, ...
its disabled by default and configurable for speed-realism
new config variables:
```
# Constant volume liquids                                                                                                                             
#liquid_finite = false                                                                                                                                
# Update liquids every .. recommend for finite: 0.2                                                                                                   
#liquid_update = 1.0                                                                                                                                  
# When finite liquid: relax flowing blocks to source if level near max and N nearby source blocks, more realistic, but not true constant.. values: 0,1,2,3,4 : 0 - disable, 1 - most aggresive                                                                                               
#liquid_relax = 1                                                                                                                                     
# optimization: faster cave flood (and not true constant)                                                                                                                    
#liquid_fast_flood = 1
```
module to make springs (infinity sources) (too destructive when placing above sea level!) https://gist.github.com/proller/4987328

testing server (can use any fresh client): h4.setun.net:30000