Declare asynchronous executor

[philippjfr]

Allows declaring an asynchronous executor on param.parameterized.async_executor which allows using async functions (i.e. coroutines) as .param.watch callbacks and will schedule them on the event loop (Tornado or asyncio). As an example, Panel will register these executors depending on whether it is running on Bokeh server or the notebook:

Notebook

param.parameterized.async_executor = IOLoop.current().add_callback

Server

param.parameterized.async_executor = pn.state.curdoc.add_next_tick_callback