MemEnv: Replace manual refcounting with scoped_refptr
MemEnv: Replace manual refcounting with scoped_refptr

This is a small cleanup that makes the code shorter and easier to read.
There are no functional changes.

Change-Id: Id854002c2f9de5dce257f1af999b4f89ae28fc63
Reviewed-on: http://gerrit.cloudera.org:8080/1438
Tested-by: Internal Jenkins
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@cloudera.com>

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