Replace LocklessAllocator with new MemoryPool class

MemoryPool maintains a free list of pre-allocated entries stored in a
libcds MPMC Queue. In contrast to LocklessAllocator, it's a lot faster,
less (and less complex) code to maintain for us and it doesn't fail when
it's full.

.gitmodules

@@ -31,6 +31,9 @@
 [submodule "src/3rdparty/mingw-std-threads"]
 	path = src/3rdparty/mingw-std-threads
 	url = https://github.com/meganz/mingw-std-threads.git
+[submodule "src/3rdparty/libcds"]
+	path = src/3rdparty/libcds
+	url = https://github.com/khizmax/libcds.git
 [submodule "doc/wiki"]
 	path = doc/wiki
 	url = https://github.com/lmms/lmms.wiki.git