This change implements thread based worker on windows.
The handling of exception from within threads will happen in separate diff.
The worker is for now used in mercurial/merge.py and in lfs extension
After multiple tests and milions of files materiealized, thousands lfs fetched
it seems that neither merge.py nor lfs/blobstore.py is thread unsafe. I also
looked through the code and besides the backgroundfilecloser (handled in base
of this) things look good.
The performance boost of this on windows is
~50% for sparse --enable-profile
- Speedup of hg up/rebase - not exactly measured