Restore context for all read methods.

Prior to 2db0aceb32 this was being done
properly, but the refactor removed stack_context.wrap causing some issues
in our upstream context managers.

tornado/iostream.py

@@ -366,7 +366,7 @@ class IOStream(object):
 
     def _set_read_callback(self, callback):
         assert not self._read_callback, "Already reading"
-        self._read_callback = callback
+        self._read_callback = stack_context.wrap(callback)
 
     def _try_inline_read(self):
         """Attempt to complete the current read operation from buffered data.