[DM] ZODB pack optimization

Implement an idea by Jim Fulton: let the first packing phase (the proper packing) be executed in a separate process with a modified reachability analysis. The modified approach prepares reachability analysis during the pack index determination. During this sequential scan all references outgoing from the scanned state are recorded in memory. The reachability analysis itself is then performed on the recorded information -- without further accesses to the storage file. This almost eliminates random access to the storage file (backpointers may still cause some randomness). This can reduce the time for reachability analysis by up to 90 percent.