add mutable Catalog::database() accessor
add mutable Catalog::database() accessor

reneme/cvmfs,djw8605/cvmfs,trshaffer/cvmfs,trshaffer/cvmfs,reneme/cvmfs,Moliholy/cvmfs,cvmfs/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,djw8605/cvmfs,alhowaidi/cvmfsNDN,DrDaveD/cvmfs,Gangbiao/cvmfs,trshaffer/cvmfs,trshaffer/cvmfs,reneme/cvmfs,alhowaidi/cvmfsNDN,cvmfs/cvmfs,cvmfs/cvmfs,Moliholy/cvmfs,DrDaveD/cvmfs,trshaffer/cvmfs,cvmfs/cvmfs,Gangbiao/cvmfs,cvmfs/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,djw8605/cvmfs,djw8605/cvmfs,alhowaidi/cvmfsNDN,reneme/cvmfs,alhowaidi/cvmfsNDN,DrDaveD/cvmfs,DrDaveD/cvmfs,Gangbiao/cvmfs,djw8605/cvmfs,Gangbiao/cvmfs,Moliholy/cvmfs,Moliholy/cvmfs,Moliholy/cvmfs,alhowaidi/cvmfsNDN,cvmfs/cvmfs,reneme/cvmfs,Gangbiao/cvmfs