[MAJOR] migrated task, tree64 and session to pool2

task and tree64 are already very close in size and are merged together.
Overall performance gained slightly by this simple change.
diff --git a/src/appsession.c b/src/appsession.c
index 72b7e40..bf26510 100644
--- a/src/appsession.c
+++ b/src/appsession.c
@@ -111,7 +111,7 @@
 	static int initialized = 0;
 	struct task *t;
 	if (!initialized) {
-		if ((t = pool_alloc(task)) == NULL)
+		if ((t = pool_alloc2(pool2_task)) == NULL)
 			return -1;
 		t->wq = NULL;
 		t->qlist.p = NULL;