Add emerge.cpp, initial EmergeThread changes

- 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

src/environment.h

@@ -40,7 +40,6 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "mapnode.h"
 #include "mapblock.h"
 
-class Server;
 class ServerEnvironment;
 class ActiveBlockModifier;
 class ServerActiveObject;