feat(pyramid-ctl): add "nb-threads" parameter to cubicweb-ctl pyramid.

This parameter will be passed to waitress.
https://docs.pylonsproject.org/projects/waitress/en/stable/arguments.html

related: #119

cubicweb/pyramid/pyramidctl.py

@@ -124,6 +124,10 @@ class PyramidStartHandler(InstanceCommand):
           'metavar': 'N',
           'help': 'Dump profile stats to ouput every N requests '
                   '(default: 100)'}),
+        ('nb-threads',
+         {'type': 'int',
+          'default': 4,
+          'help': 'The number of threads used by the server.'}),
         ('param',
          {'short': 'p',
           'type': 'named',
@@ -291,7 +295,7 @@ class PyramidStartHandler(InstanceCommand):
         repo = app.application.registry['cubicweb.repository']
         try:
             waitress.serve(app, host=host, port=port, url_scheme=url_scheme,
-                           clear_untrusted_proxy_headers=True)
+                           clear_untrusted_proxy_headers=True, threads=self['nb-threads'])
         finally:
             repo.shutdown()
         if self._needreload: