rculfhash: Move key out of struct lfht_test_node, move key_len to caller

Make struct lfht_test_node only contains basic field.
Let user take the responsibility to handle the <key>(hash_set)
or <key,value>(hash_map) management and calculation.

[ Edit by Mathieu Desnoyers: rebased on top of commit
  0422d92c2d658f6093b8209f75808efd2109a110 ]

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>