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/src/pipe.c b/src/pipe.c
index 8c82821..6fe7b05 100644
--- a/src/pipe.c
+++ b/src/pipe.c
@@ -15,7 +15,7 @@
 
 #include <haproxy/api.h>
 #include <haproxy/thread.h>
-#include <common/memory.h>
+#include <haproxy/pool.h>
 
 #include <types/global.h>
 #include <types/pipe.h>