CLEANUP: pools: rename all pool functions and pointers to remove this "2"

During the migration to the second version of the pools, the new
functions and pool pointers were all called "pool_something2()" and
"pool2_something". Now there's no more pool v1 code and it's a real
pain to still have to deal with this. Let's clean this up now by
removing the "2" everywhere, and by renaming the pool heads
"pool_head_something".
diff --git a/include/proto/session.h b/include/proto/session.h
index 7f95e2a..d63d29e 100644
--- a/include/proto/session.h
+++ b/include/proto/session.h
@@ -32,7 +32,7 @@
 
 #include <proto/stick_table.h>
 
-extern struct pool_head *pool2_session;
+extern struct pool_head *pool_head_session;
 struct session *session_new(struct proxy *fe, struct listener *li, enum obj_type *origin);
 void session_free(struct session *sess);
 int init_session();