Revert "Revert "hashtable: introduce a small and naive hashtable""

This reverts commit 4fde1ece6172dcd40fd84bf859948f99f1948226.
Revert "Revert "block,elevator: use new hashtable implementation""

This reverts commit 878c871431374cd56b1c51da699ba5ee70158a08.
Upate to linux 3.4.63