Make setup async to get rid of setup()

[olsaarik]

This PR changes the Bootstrap API to make recv calls asynchronous (they return a std::future<void>) and implements this change in TcpBootstrap by moving receives to background threads. For each pair of source rank and tag there is a thread that performs the receives in the order they were issued.

recv is marked as [[nodiscard]] to avoid ignoring the asyncness of the function. However, I didn't see a warning from that in my build and I'm not sure why.

The NonblockingFuture class is now gone in favor of std::future everywhere. BaseSemaphore has a std::shared_future because there are multiple calls to get in its current form and std::future turns invalid after the first call to get. It might make sense to redesign the semaphores API to return semaphores in a std::future, so that after initialization the .get() indirection is avoided.

Alternatively, we could make everything std::shared_future to make the APIs overall easier to use, although that might feel a bit prescriptive. std::future can be a bit surprising especially on the Python side where calling .get() multiple times doesn't feel like it should be an error.

As an unrelated change this PR renames getRank to rank and getNRanks to size to conform to C++ and MPI naming.

Finally, instead of fixing the existing Python examples I moved the bootstrap example into a Jupyter notebook in the docs/ directory. The other one didn't seem like it's worth keeping anymore since we have the Python tests now.

[chhwang]

Looks great!

[Binyang2014]

LGTM

[saeedmaleki]

this looks great but is there any potential for slowdowns?

[chhwang]

@olsaarik I have made a few updates to remove setup() in Python tests: 1773207, but some tests such as test_connection_write are still failing. Could you please have a look?