ansible: pin connection multiplexer to a single core

Nets a reliable 8% improvement in issue_140__thread_pileup.yml when
targetting 64 Docker containers on the same 8 core/16 thread machine.

Before:
    2294528.731458      task-clock (msec)         #    6.443 CPUs utilized
        10,429,745      context-switches          #    0.005 M/sec
         2,049,618      cpu-migrations            #    0.893 K/sec
         8,258,952      page-faults               #    0.004 M/sec
 5,532,719,253,824      cycles                    #    2.411 GHz                      (83.35%)
 4,001,276,805,120      stalled-cycles-frontend   #   72.32% frontend cycles idle     (83.30%)
 2,024,159,442,463      stalled-cycles-backend    #   36.59% backend cycles idle      (66.65%)
 3,267,471,616,230      instructions              #    0.59  insn per cycle
                                                  #    1.22  stalled cycles per insn  (83.35%)
   662,006,455,943      branches                  #  288.515 M/sec                    (83.33%)
    39,453,895,977      branch-misses             #    5.96% of all branches          (83.37%)

     356.148064576 seconds time elapsed

After:
    2208247.938562      task-clock (msec)         #    6.735 CPUs utilized
         8,489,840      context-switches          #    0.004 M/sec
         1,432,967      cpu-migrations            #    0.649 K/sec
         7,508,957      page-faults               #    0.003 M/sec
 5,477,293,750,357      cycles                    #    2.480 GHz                      (83.31%)
 3,984,360,350,811      stalled-cycles-frontend   #   72.74% frontend cycles idle     (83.32%)
 1,976,646,418,711      stalled-cycles-backend    #   36.09% backend cycles idle      (66.64%)
 3,196,197,480,792      instructions              #    0.58  insn per cycle
                                                  #    1.25  stalled cycles per insn  (83.36%)
   648,247,332,967      branches                  #  293.557 M/sec                    (83.35%)
    39,004,881,070      branch-misses             #    6.02% of all branches          (83.37%)

     327.876903668 seconds time elapsed

ansible_mitogen/process.py

@@ -176,6 +176,7 @@ def start(cls, _init_logging=True):
             cls.child_sock = None
             mitogen.core.io_op(cls.worker_sock.recv, 1)
         else:
+            mitogen.utils.reset_affinity()
             cls.worker_sock.close()
             cls.worker_sock = None
             self = cls()