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/src/tcp_rules.c b/src/tcp_rules.c
index c72afb0..feee441 100644
--- a/src/tcp_rules.c
+++ b/src/tcp_rules.c
@@ -214,7 +214,7 @@
 					goto missing_data;
 
 				if (cap[h->index] == NULL)
-					cap[h->index] = pool_alloc2(h->pool);
+					cap[h->index] = pool_alloc(h->pool);
 
 				if (cap[h->index] == NULL) /* no more capture memory */
 					continue;