sanlock: fix release interference with paxos
    
    The process of releasing a lease on disk involved
    first zeroing our own dblock values prior to zeroing
    the leader record.  Clearing dblock values can
    interfere with an in-progress paxos ballot, and can
    result in a second host owning the same lease version
    that we released.
    
    The incorrect zeroing of dblocks was added in
    commit d6bef45b9716c581d99466280a52a01c9ebe3bf7
    
    The reason for zeroing was to fix an issue with shared
    leases here: e40e1f6e22f9b10f08d53fc7da94f5158d9e4ae8
    That issue is now resolved by setting a new flag in the
    dblock to indicate we are releasing the lease.  The new
    flag is set, along with the unchanged dblock values,
    before clearing the leader block.