Eliminate drbbdup bookkeeping for single-case-forever uses

With drbbdup inside drmemtrace (#3995) we're seeing drbbdup use an extra ~35MB ("Peak vmm virtual memory in use" rstat) for ~150K blocks; so ~140MB for ~600K blocks. This is not horrible, but is not insignificant, and is unnecessary: if the user promises to never use more than just one case (which memtrace can certainly do based on its own runtime options at init time) drbbdup shouldn't need any bookkeeping.