Optimized import

Hi! I worked on optimizing import. As a side effect this also reduces replication and event handling (devpi-web indexing) times, because fewer serials are generated. A development release is available here: https://devpi.net/fschulze/dev/+f/fc1/506ba44a0f4b9/devpi-server-3.1.0.dev0.... The PR with the changes is here: https://bitbucket.org/hpk42/devpi/pull-requests/347/optimize-import/diff Regards, Florian Schulze