Changeset import: Performance improvements via bulk insert

Processing about 21 Mio. changesets one by one seemed to take quite a lot of time. This is an attempt to create packages of up to 5000 changesets and insert them at once. This brought down import times down to 3-4 hours on my machine. This may still not be perfect, but good enough for me. See README.md changes for more details.

Please review carefully before applying as I'm really a Ruby n00b...