master: cache sent/forwarded module names

On 32x Docker run of issue_140__thread_pileup.yml

Before:

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
   384807    3.595    0.000   12.207    0.000 /home/dmw/src/mitogen/mitogen/master.py:896(_forward_one_module)
  1218352    4.867    0.000    7.302    0.000 /home/dmw/src/mitogen/mitogen/master.py:853(_send_module_and_related)

After:

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
   384807    3.839    0.000    6.543    0.000 /home/dmw/src/mitogen/mitogen/master.py:902(_forward_one_module)
  1218352    0.723    0.000    0.898    0.000 /home/dmw/src/mitogen/mitogen/master.py:856(_send_module_and_related)

mitogen/master.py

@@ -707,6 +707,9 @@ def __init__(self, router):
         self.blacklist = []
         self.whitelist = ['']
 
+        #: Context -> set([fullname, ..])
+        self._forwarded_by_context = {}
+
         #: Number of GET_MODULE messages received.
         self.get_module_count = 0
         #: Total time spent in uncached GET_MODULE.
@@ -846,6 +849,9 @@ def _send_module_load_failed(self, stream, fullname):
         )
 
     def _send_module_and_related(self, stream, fullname):
+        if fullname in stream.sent_modules:
+            return
+
         try:
             tup = self._build_tuple(fullname)
             for name in tup[4]:  # related
@@ -889,6 +895,14 @@ def _send_forward_module(self, stream, context, fullname):
             )
 
     def _forward_one_module(self, context, fullname):
+        forwarded = self._forwarded_by_context.get(context)
+        if forwarded is None:
+            forwarded = set()
+            self._forwarded_by_context[context] = forwarded
+
+        if fullname in forwarded:
+            return
+
         path = []
         while fullname:
             path.append(fullname)