use a common poller thread for modules sharing io

When several poller threads are using the same io, the resposivity
of client requests is reduced, as every thread first finishes
its pending communication requests, before it is the turn of the
request thread. This is solved by using one common poller thread
for all modules sharing the same communicator.

+ fix an issue with overriding a property with a parameter, as
  this is the case for pollperiod (cfg was applied to property
  instead of overriding parameter)
+ separate setFastPoll arguments into flag and fast interval
+ fix missing announceUpdate call when read function fails
+ fix mechanism for triggering polls after an io connection
  reconnected again.

Change-Id: I1115a61fae3de80d18416e61f40b52a0eebb637c
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/28021
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop/properties.py

@@ -136,10 +136,11 @@ class HasProperties(HasDescriptors):
             properties.update({k: v for k, v in base.__dict__.items() if isinstance(v, Property)})
         cls.propertyDict = properties
         # treat overriding properties with bare values
-        for pn, po in properties.items():
+        for pn, po in list(properties.items()):
             value = getattr(cls, pn, po)
-            if not isinstance(value, (Property, HasProperties)):  # attribute may be a bare value
-                # HasProperties is a base class of Parameter -> allow a Parameter to override a Property ()
+            if isinstance(value, HasProperties):  # value is a Parameter, allow override
+                properties.pop(pn)
+            elif not isinstance(value, Property):  # attribute may be a bare value
                 po = po.copy()
                 try:
                     # try to apply bare value to Property