sanlock: fix release clearing of host block
    
    This is a comprehensive fix for the regression
    caused by e40e1f6e22f9b10f08d53fc7da94f5158d9e4ae8
    which caused the dblock to be zeroed in paxos_lease_release.
    
    That commit failed to account for the fact that clearing
    the dblock also cleared the mblock (mode_block) that has
    the SHARED flag.  This broke shared leases by clearing
    the SHARED flag immediately after setting it.
    
    Clearing the dblock and mblock is now moved up to
    the resource layer above the paxos layer.  Every
    place that releases paxos leases or modifies the
    mode block now needs to choose what should be
    done with the "host block" (referring to the
    host's sector containing the dblock and mblock.)