REORG: include: move common/memory.h to haproxy/pool.h

Now the file is ready to be stored into its final destination. A few
minor reorderings were performed to keep the file properly organized,
making the various sections more visible (cache & lockless).

In addition and to stay consistent, memory.c was renamed to pool.c.
diff --git a/include/proto/session.h b/include/proto/session.h
index 522e43f..866217e 100644
--- a/include/proto/session.h
+++ b/include/proto/session.h
@@ -24,7 +24,7 @@
 
 #include <haproxy/api.h>
 #include <common/buffer.h>
-#include <common/memory.h>
+#include <haproxy/pool.h>
 
 #include <types/global.h>
 #include <types/session.h>