Changeset import: Performance improvements via bulk insert

Processing about 21 mio changesets one by one seemed to take quite a lot of time. In this pull request I've included a solution to create packages of up to 5000 changesets (configurable parameter) and insert them at once. This way import times went down to 3-4 hours on my machine. See README.md changes for more details.

Please review carefully before applying as I'm really a Ruby n00b. Code is probably not ruby idiomatic, feel free to adjust as needed.