fix, renaming and restructure of event stuff

Adding the methods necessary for Modifying Subscriptions, make deadband filters and also added support to deadband filter on server side.

default strings and ByteString to None

small renaming, mv more methods to events.py

implement dict based event history, cleanup event stuff

event history api tweak

take event as arg not evfilter
return EventResult obj and not fields
update command line to read events

Correctly handle count parameter when saving history

WhereClause support

* Added missing handling of the count parameter to the sqlite implementation
* Add tests for both limits and both history implementations
* Fixed some minor issues

The test code is currently setting a limit of 10 history data values.
However, the limit handling is not implemented in both history
implementations and therefore the limit is never enforced.
Removed the limit from the history tests to ensure that no tests are broken
when the limit handling is actually implemented.

The current implementation is actually a no-op since the data is truncated
correctly but not assigned back to self._datachanges.
Pop the first value instead since this function is called for every
datachange which means that not more than one value will every be removed
in a single call.

create a whereclause when subscribing to events, allow user to provide a custom EventFilter object as arg

Lazyloading

Introduce a new optional parameter to the constructor of the server. If
a cache file path is defined, the address space will be loaded from the
cache or the cache file will be created if it dose not exist yet.