sync/Jupyter: don't create too many patches

[williamstein]

The backend for Jupyter can create several snapshots per second, e.g., when flushing output buffers. This leads to issues such as #6280 . Put a hard limit on this.

[williamstein]

I.e., have a limit of at most 2 patches per second.

Here is an example to illustrate the problem:

import sys, time
for i in range(100):
    sys.stdout.write("%s"%i)
    sys.stdout.flush()
    time.sleep(0.1)

Yes, this will make output like this much less smooth. But that's fine and what something ipywidgets addresses properly.

[williamstein]

In case anybody finds this and wants smooth output without buffering, use an output widget.

For example:

import ipywidgets as widgets
out = widgets.Output(); out

then

for i in range(100):
    out.append_stdout(str(i))
    time.sleep(0.1)

to get this with fast smooth output: