More parallellization in the batch importer

- Node import stage and calculating dense nodes stage are now run in
  parallell
- All I/O writing is done by IoQueue, which is a separate thread pool with
  configurable amount of threads and a funnel into that pool per store file.

The above have been seen to increase performance by 4x.
IdMapper is an interface

renamed from NodeIdMapper, now used directly in the importer, instead of
the NodeIdMapping indirection which was an enum.
Some refactorings around producers
Merge pull request #2653 from tinwelint/2.1-parallellize-more-batch-import

2.1 parallellize more batch import