workers: add config to enable/diable workers This adds config to disable/enable workers with default being enabled. Test Plan: enabled profile without updaing .hg/hgrc (the default should be to use workers) and ran hg sprase --enable-profile <profile>.sparse Watched in the proces explorer that hg started 12 new threads for materializing files (this is my worker.numcpus) value Added [worker] enabled = False to the .hg/hgrc and re ran the command. This time hg didn't spawn any new threads for matreializing of files Differential Revision: https://phab.mercurial-scm.org/D1460