avoid lost messages after activate message

messages might get lost after activate, when the number of parameters
exceeds maxlen of the output queue, as a limited deque silently overwrites
messages.

first try: use an infinite deque, but slow down appending thread when deque
gets too big
-> does not work because the appending thread is the same

second try: increase maximum a lot, on overflow log an error message and
close connection

Change-Id: I20376f7e08240dabe43269fa63c596f07e59ddf6
Reviewed-on: https://forge.frm2.tum.de/review/20982
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop/protocol/interface/__init__.py

@@ -32,8 +32,10 @@ def encode_msg_frame(action, specifier=None, data=None):
     data may be an json-yfied python object"""
     action = action.encode('utf-8')
     if specifier is None:
-        # implicit: data is None
-        return b''.join((action, EOL))
+        if data is None:
+            return b''.join((action, EOL))
+        # error_activate might have no specifier
+        specifier = ''
     specifier = specifier.encode('utf-8')
     if data:
         data = json.dumps(data).encode('utf-8')