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.

metrics

Perform lazy loading when restoring a cached address space

Fixing the Modify Monitored Item response in the server.

When starting an opcua server, the creation of the address space is
currently a performance bottleneck. The startup process can be accelerated
by loading a pre-generated address space pickle.
However, the startup process still takes ~25 seconds on a raspberry pi
model b (compared to ~125 seconds when generating the address space from code).
Storing the address space in a shelve, where the data for each node is
pickeled individually, allows to further improve the startup performance
since only the nodes that are actually accessed are loaded from
disc (all other nodes are loaded later when they are accessed).
Since the default address space contains thousands of nodes but just a
small amount is actually accessed during startup, the startuptime could
be improved to ~3.5 seconds.

Events History with Tests

Handle binary objects to be stored in the database better.

Clarify if method is history for data change, or events.
Rename get_field_variants method and remove string decode hack.
Add logger to history manager and improve dehistorize
Update history tests so that only SQL tests handle events.
Minor doc string updates.