sanlock: reuse resource structs
    
    Keep unused resource structs on a list and reuse
    them if the same resource is requested again.
    This is meant to keep the resource id (rN) the
    same instead of increasing for every request.
    The limited number of unused saved structs means
    that the same resource can get a new id between
    requests.  Internally, the token_id (unique for
    each token/request) is replaced by the res_id.