- Neatly placed all emerge related code into a new file, emerge.cpp
- Greatly cleaned up the code in EmergeThread::Thread()
- Reworked Emerge queue.  Now an actual std::queue of v3s16 block positions
- Removed the completely unnecessary map of peer ids requesting blocks

This causes glitches in very high cliffs by not making mud "flow" down from
them. Those are quite rare currently because of the lame height differences
generated.

Switch the license to be LGPLv2/later, with small parts still remaining as GPLv2/later, by agreement of major contributors