Prevent fluids from flowing outside the island

[Poslovitch]

Is your feature request related to a problem? Please describe.
Fluids flowing outside the island can result in cobblestone/stone/obsidian being formed in the area between two islands' protection ranges, thus unbreakable by neighbouring players.

Describe the solution you'd like
A WORLD_SETTING flag that'd prevent this.

Additional context
It may be a performance intensive flag, if enabled. However, I think it's better to set it to true by default.
According to some server owners, PlotSquared has this feature - we may want to have a look at it and see how they did.

[tastybento]

I did this in Greenhouses: https://github.com/BentoBoxWorld/Greenhouses/blob/master/src/main/java/world/bentobox/greenhouses/listeners/GreenhouseGuard.java

You have to have really efficient way to check in or out of an island because these flows can occur continuously. Also, you may want to not check any y-coordinate flows only, just x and z differences.

[Poslovitch]

Wow, it took me quite a long time to get it working, but here are the results!

It is blocked at the edge of the protected range:

As you can see, water can still flow vertically, even next to the protected range:

I'm however encountering this issue when the water source is next to the protected range: it will flow vertically, but not sideways:

And, for reference, a water block in the area between islands remain still - but it can flow vertically as well:

I won't be fixing this bug unless asked, there's no proper API to make it work correctly right now.
I will however write a unit test before committing the code.

[Poslovitch]

And done!

Writing test units is very time consuming... Hope it'll do its job and help us avoid any sneaky regression in the future!