Configurable throttling schemes on ServerCallback

[philippjfr]

The throttling scheme used by LinkedStream callbacks on bokeh server has never really worked very robustly. This latest attempt fixes the existing throttle mode and adds two additional modes so that now we support the following throttling modes:

• adaptive (default): The callback adapts the throttling timeout depending on the rolling mean of the time taken to process each message.
• throttle: Uses the fixed throttle_timeout as the minimum amount of time between events.
• debounce: Processes the message only when no new event has been received within the throttle_timeout duration.

I would like to come up with a good way to expose these options.

• Also fixes unlock mechanism for bokeh document is not working by default #4371 by ensuring server updates are processed immediately as long as the Document can be unlocked