workers: implemented worker on windows 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 Test Plan: Ran 10s of hg sparse --enable-profile and --disable-profile operations on large profiles and verified that workers are running. Used sysinternals suite to see that all threads are spawned and run as they should Run various other operations on the repo including update and rebase Ran tests on CentOS and all tests that pass on @ pass here Differential Revision: https://phab.mercurial-scm.org/D1458