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/CMakeLists.txt

@@ -209,6 +209,7 @@ set(common_SRCS
 	script.cpp
 	log.cpp
 	content_sao.cpp
+	emerge.cpp
 	mapgen.cpp
 	mapgen_v6.cpp
 	treegen.cpp