concurrent_btree: fix memory leak of root node
concurrent_btree: fix memory leak of root node

Was previously allocating root node outside the arena,
but in freeing the structure, assumed all leaf nodes
came from the arena

EvilMcJerkface/kudu,helifu/kudu,cloudera/kudu,cloudera/kudu,InspurUSA/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu,andrwng/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,cloudera/kudu,EvilMcJerkface/kudu,andrwng/kudu,cloudera/kudu,cloudera/kudu,EvilMcJerkface/kudu,cloudera/kudu,andrwng/kudu,andrwng/kudu,InspurUSA/kudu,InspurUSA/kudu,andrwng/kudu,helifu/kudu,cloudera/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,helifu/kudu,helifu/kudu,andrwng/kudu,cloudera/kudu,helifu/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu,andrwng/kudu,InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,andrwng/kudu,helifu/kudu,andrwng/kudu,cloudera/kudu,EvilMcJerkface/kudu,InspurUSA/kudu