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.

This commit is very functional, but WhereClause is not supported yet.
Default history will fail tests because it raises NotImplemented on
event calls.

Event implementation moved to server

Example update and new client-events example

Events fixing

Events implementation proposal

New-style classes

Unless you are doing some hack that requires old-style classes, they should be new-style ones to ensure forward compatibility.