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/map.h

@@ -46,6 +46,8 @@ class MapBlock;
 class NodeMetadata;
 class IGameDef;
 class IRollbackReportSink;
+class EmergeManager;
+class BlockMakeData;
 
 
 /*
@@ -378,7 +380,7 @@ public:
 		Blocks are generated by using these and makeBlock().
 	*/
 	void initBlockMake(BlockMakeData *data, v3s16 blockpos);
-	MapBlock* finishBlockMake(BlockMakeData *data,
+	MapBlock *finishBlockMake(BlockMakeData *data,
 			core::map<v3s16, MapBlock*> &changed_blocks);
 
 	// A non-threaded wrapper to the above  - DEFUNCT