issue #369: fix KeyError during new context start.

Update _via_by_context earlier; fixes:

    Traceback (most recent call last):
      File "/Users/dmw/src/mitogen/mitogen/service.py", line 519, in _on_service_call
	return invoker.invoke(method_name, kwargs, msg)
      File "/Users/dmw/src/mitogen/mitogen/service.py", line 253, in invoke
	response = self._invoke(method_name, kwargs, msg)
      File "/Users/dmw/src/mitogen/mitogen/service.py", line 239, in _invoke
	ret = method(**kwargs)
      File "/Users/dmw/src/mitogen/ansible_mitogen/services.py", line 454, in get
	reraise(*result)
      File "/Users/dmw/src/mitogen/ansible_mitogen/services.py", line 412, in _wait_or_start
	response = self._connect(key, spec, via=via)
      File "/Users/dmw/src/mitogen/ansible_mitogen/services.py", line 363, in _connect
	self._update_lru(context, spec, via)
      File "/Users/dmw/src/mitogen/ansible_mitogen/services.py", line 266, in _update_lru
	self._update_lru_unlocked(new_context, spec, via)
      File "/Users/dmw/src/mitogen/ansible_mitogen/services.py", line 253, in _update_lru_unlocked
	if self._refs_by_context[context] == 0:
    KeyError: Context(1008, u'ssh.localhost.sudo.mitogen__user3')

ansible_mitogen/services.py

@@ -244,6 +244,8 @@ class ContextService(mitogen.service.Service):
         by `kwargs`, destroying the most recently created context if the list
         is full. Finally add `new_context` to the list.
         """
+        self._via_by_context[new_context] = via
+
         lru = self._lru_by_via.setdefault(via, [])
         if len(lru) < self.max_interpreters:
             lru.append(new_context)
@@ -257,7 +259,6 @@ class ContextService(mitogen.service.Service):
                         'but they are all marked as in-use.', via)
             return
 
-        self._via_by_context[new_context] = via
         self._shutdown_unlocked(context, lru=lru, new_context=new_context)
 
     def _update_lru(self, new_context, spec, via):