Fix a bug in the dirty page writeback code of the disk hash index.
Changes/Fixes diff handling of write-back cache
Background committer should use the optimized write-back method.
Add unit test to check chunk index entry importing

Checks the chunk index importing in the absense of a log replay.
Add resume handle so that chunk index bg imports resume at some bucket.
Add unsafe disk hash index mode.

This allows a higher performance if crash safety is not necessary.
Fix write back page counting
Add TODO comment to block index.

It is not clear why the block index importing stops at 16K blocks and
then waits for the log replay.
Reduce the default bg import thread count to 1.
Fixes a unit test.