Prevent ui.leaflet from flickering after calling flyTo()

[falkoschindler]

This PR attempts to solve #3035 by adding a short delay on client and server to allow both zoom and center to update without jumping back and forth. The delay of 0.02s is deliberately chosen above the lower bound of 0.016s for Windows. I hope it isn't too dependent on processor and network speed.

Can be tested with this code:

m = ui.leaflet(center=(48.1, 11.6), zoom=10)
m.on('map-moveend', lambda e: print(e.args['center'], e.args['zoom']))
ui.button('Berlin', on_click=lambda: m.run_map_method('flyTo', [52.5, 13.4], 9, {'duration': 1.0}))

There might still be glitches with multiple tabs connected to the auto-index page. But maybe it's good enough for now.

[afullerx]

This fix works well on Windows. I did my best to break it by opening the UI in multiple tabs, maxing out the CPU, and throttling the network to "slow 3G". In all cases it continued to function nicely. 👍