update some examples, make them work and add several missing async methods

remove unnecessary assert. the length of results might be different from the length of arguments, for example for delete since we are recursive

!! Warning, API-breaking changes!

When closely reading the spec, events are emitted by one node, and refer
to their SourceNode in an attribute — the SourceNode can and often will
be different from the emitting node.

Until now, python-opcua assumed these two to be identical. This commit
tries to straighten this situation.

To that end, we introduce new parameters in the eventgenerator,
Event-class and all children thereof. The EventGenerator will now try to
honor emitting nodes given to it, but still handle and add SourceNodes.
The InternalSubscription finally serializing and emitting the event is
adapted to this new behavior.

See https://github.com/FreeOpcUa/python-opcua/issues/755
This fixes inconsistent Encoding types when performing
y=variant_to_binary(x) -> x2=variant_from_binary(y) on
auto-generated attributes in the standard address space.

Certificate is only used at higher Server level, no need to store in InternalServer

to_binary() was replaced by variant_to_binary as of commit e1067bac

Deleting a node on which any client holds a datachange-notification
subscription, NodeManagementService._delete_node_callbacks fails because
the size of the dictionary it loops over changes. Here, we solve this by
not using the .items() iterator, but making a copy in form of a list
first ….

shoud be of type Int32

This reverts commit b934a49b.

NotificationData is sometimes None. And that causes an exception that breaks
the flow and no SubscriptionAcknowledgement is created and published
to the server.