Synchronously create Document patch message to avoid race conditions

[philippjfr]

In order to support immediate dispatch of events when a Document lock is not required we have a special handler that sends messages on the websocket when Bokeh would ordinarily wait to acquire a lock explicitly. When a document lock is required, however, this custom handling can result in race conditions because the event isn't dispatched until long after the model has been added as a reference to the Document, meaning that when the message is created and dispatched it assumes the model has already been fully serialized. In order to work around this we immediately serialize events into messages ensuring that they correctly reflect the state of the document at the time the event is generated.

Fixes #6019