Simplifies window pool factory management in batch importer

by exposing a way to change the mode (APPEND/UPDATE) instead of closing
and recreating a new neostore. Also fixes an issue where there might have
still been queued write jobs not executed while starting to execute stages
that read the stores starting from the end.
Merge pull request #2781 from tinwelint/2.1-batchimport-ioqueue-issue

Simplifies window pool factory management in batch importer