Bring Python API up-to-speed with recent C++ features

[pentschev]

• Add ucxx::MemoryHandle and ucxx::RemoteKey C++ classes #190 added support for UCX memory handles and remote keys;
• Add support for RMA over UCP #166 added support for UCX RMA;
• Add Active Message receiver callbacks #186 adds support for active messages receiver callbacks;
• Add non-blocking ucxx::Endpoint::close() request #192 adds support for async endpoint closing requests.

[DrBinary9]

@pentschev Hi, I'm ready to take up these issue. Which one should I start with?

[pentschev]

Hi @DrBinary9 , I was having a look and actually none of them is going to be relatively simple, all of them have at least a dozen of methods that need to be added and tested. If you still want to get started, probably the best choice would be #192, primarily because I already added a basic part of it (namely the close_blocking method). Nevertheless, what still need to be done is (if I haven't forgotten anything):

1. Add new methods to the Python API
2. Add tests that cover the same cases as the C++ tests

I linked above only the changes from that PR, but there may have been new changes since then that modify either the API or tests, thus whatever is the new state is what should be covered, and not exactly the changes linked.

The process of adding new methods involves:

1. Adding missing prototypes in ucxx_api.pxd
2. Expose them to the Python core API in libucxx.pyx
3. Add tests to the Python core API
4. Expose them to the Python async API in endpoint.py
5. Add tests to the Python async API

This is all a lot of work, so please make sure you're really willing to spend that much time working on all these changes, they are expected to be very time consuming.

If you're experienced/interested in the C++ part, there are many simpler tasks to be done than this.