`shm` transport causes tests to hang

[pentschev]

Some of the larger tests below will hang indefinitely when shm is enabled, but removing that transport and leaving only tcp succeeds, enabling other transports such as rc and posix is also fine.

UCX_TLS=shm,tcp,cuda_copy pytest -vs --cache-clear tests/test_send_recv.py

It's not clear whether this is a bug in UCX, but it's possible that support for shm in blocking mode isn't stable. We need to test #565 again.

[pentschev]

This seems to be working with UCX master:

$ ucx_info -v
# UCT version=1.12.0 revision af82548
# configured with: --disable-logging --disable-debug --disable-assertions --disable-params-check --prefix=/datasets/pentschev/miniconda3/envs/rn-112-21.12.211025 --with-cuda=/usr/local/cuda-11.2 --enable-mt
$ UCX_TLS=tcp,cuda_copy,shm pytest tests/test_send_recv.py
================================================= test session starts ==================================================
platform linux -- Python 3.8.12, pytest-6.2.5, py-1.10.0, pluggy-1.0.0
rootdir: /datasets/pentschev/simple-ci/src/ucx-py
plugins: asyncio-0.16.0, repeat-0.8.0, rerunfailures-10.2
collected 146 items

tests/test_send_recv.py ........................................................................................ [ 60%]
..........................................................                                                       [100%]

================================================= 146 passed in 45.12s =================================================

cc @madsbk