Add safety checks when running GC on repo gateway
Add safety checks when running GC on repo gateway

* Do not run GC if there are active leases
* Stop cvmfs-gateway at the beginning of the GC run and  restore it at the end

DrDaveD/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs