Allow writing to Streamlit from multiple threads

[tvst]

See a possible solution here: https://gist.github.com/tvst/fa33b9dcb58040cbcb0ea376146d4e8c
and see https://discuss.streamlit.io/t/live-plot-from-a-thread/247 for context.

A few thoughts on that Gist:

1. Can we make that API match the one from st? That is, it would be great if methods from the main DeltaGenerator were accessible as thread_safe_st.foo rather than from thread_safe_st.main.foo
2. Even better, can the user just pass the st object itself to the other thread? Why do we need ThreadSafeSt at all?
3. If it turns out that a ThreadSafeSt is needed, we should come up with a better name for it. Something likeStreamlitContext sounds good to me, but we already have a ReportContext in our codebase, so it may be confusing. Maybe rename ReportContext in the process?

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[caspark]

Seems like it's pretty straightforward to address point 2 by extending ThreadSafeSt like this:

class ThreadSafeSt():
    # ... original class body

    def __getitem__(self, item):
        return self.main[item]

    def __getattr__(self, item):
        return getattr(self.main, item)

Haven't tested it extensively, but it seems to work well enough for my purposes!