ReadOp.GetOmapValuesByKeys() now uses rados_read_op_omap_get_vals_by_keys2().
GetOmapSte.Next() and ReadOpOmapGetValsByKeysStep.Next() now use rados_omap_get_next2().
This commit implements binding for read_op_omap_get_vals_by_keys RADOS Read operation. Includes a unit test. Signed-off-by: Robert Vasek <robert.vasek@cern.ch>