- Add getRecordCount() to TunnelBuildMessage and TunnelBuildReplyMessage
        so they can be extended.
      - New I2NP Messages VariableTunnelBuildMessage and VariableTunnelBuildReplyMessage,
        which contain the number of request slots in them.
      - Convert all static assumptions of 8 slots to getRecordCount()
      - Use the new VTBM if all hops in the tunnel and the OBEP or IBGW of the reply tunnel
        support it, and the tunnel is 4 hops or shorter.
      - Reply to a VTBM with a VTBRM of the same size
      - Make BuildReplyHandler static
      - Convert the currentlyBuilding List to a ConcurrentHashMap to speed reply lookups
        and eliminate a global lock; don't put fallback tunnels in there
      - Add new tunnel.corruptBuildReply stat
      - Various cleanups and javadoc

    Tested as compatible with current network, new messages untested.