Batch importer actually flushes the current window when told to

this to support proper shutdown of the neostore, where pending changes are
flushed and writes awaited so that the store can be closed properly.
Fixes relationship augmentation when node has self-relationships
Merge pull request #2787 from tinwelint/2.1-flush-bip-windowpool

Batch importer actually flushes the current window when told to
Merge pull request #2784 from jexp/rel-augmentation-loop

Fixes relationship augmentation when node has self-relationships