Attempt to fix the big bug. Now server either stops sending map or mapgen starts generating CONTENT_IGNORE.

* liquid flow no longer does random things when transformed
* if a flowi
ng liquid node doesn't have a solid block as neighbor, its spread distan
ce is shortened by 1

Completely rewrote the liquid transformation. The algorithm now exclusively modifies the current node in one iteration. Another change is that MapNode.param2 now stores a bit that tells other nodes if a flowing liquid node flows downwards. This is accomplished by two masks on param2 for the "flow down" bit and the "liquid level" bits.

This will be the base of future improvements to the liquid flow. However, in the current state flowing liquid does not always disappear when cut off its source. Be aware that this branch is work in progress.

A small fix to an unlikely situation where the map generator requires a block that hasn't been loaded from disk. Now it is actually loaded from disk in that case.

- When flowing liquid starts to flow down, it stops contributing to its neighboring nodes' liquid level (possibly transforming it back to air)
- Flowing liquid turns into a source if there are at least 2 adjacent liquid source nodes
- If a new liquid flow reaches existing flowing liquid, the existing liquid is now updated properly.

Fixed objects being sometimes not able to be stored statically in a block when block has been unloaded

New map generator added (and SQLite, messed up the commits at that time...) (import from temporary git repo)

Moved some mapnode content stuff from mapnode.{h,cpp} and digging property stuff from material.cpp to content_mapnode.{h,cpp}

Added a check that avoids server crash (unhandled exception) when map generation is attempted near limit