Speed up change notifications

[teusbenschop]

Generating the change notifications of e.g. importing a whole Bible takes a lot of time.

When generating the changes per user, there's no feedback. It would be better if feedback is give, like per chapter.

But speeding up generating changes is much desired. C++ needs to be written much more efficient so there's no time lost in copying data around, but rather to use existing stores of data.