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

mercurial/configitems.py

@@ -1253,6 +1253,9 b" coreconfigitem('worker', 'backgroundclos"
 coreconfigitem('worker', 'backgroundclosethreadcount',
     default=4,
 )
+coreconfigitem('worker', 'enabled',
+    default=True,
+)
 coreconfigitem('worker', 'numcpus',
     default=None,
 )

mercurial/help/config.txt

@@ -2563,6 +2563,10 b' Parallel master/worker configuration. We'
 directory updates in parallel on Unix-like systems, which greatly
 helps performance.
 
+``enabled``
+    Whether to enable workers code to be used.
+    (default: true)
+
 ``numcpus``
     Number of CPUs to use for parallel operations. A zero or
     negative value is treated as ``use the default``.

mercurial/worker.py

@@ -82,7 +82,8 b' def worker(ui, costperarg, func, statica'
     args - arguments to split into chunks, to pass to individual
     workers
     '''
-    if worthwhile(ui, costperarg, len(args)):
+    enabled = ui.configbool('worker', 'enabled')
+    if enabled and worthwhile(ui, costperarg, len(args)):
         return _platformworker(ui, func, staticargs, args)
     return func(*staticargs + (args,))