Spead2

- Ensure correct operation when `loop=None` is passed explicitly to trollius
stream constructors, for consistency with functions that have it as a keyword
parameter.

- Suppress ``recvmmsg: resource temporarily unavailable`` warnings (fixes 43)

- Extend :py:class:`~spead2.MemoryPool` to allow a background thread to
replenish the pool when it gets low.
- Extend :py:class:`~spead2.ThreadPool` to allow the user to pin the threads to
specific CPU cores (on glibc).

- Fix ring_stream destructor to not deadlock (fixes 41)

- Change handling of incomplete heaps (fixes 39). Previously, incomplete heaps
were only abandoned once there were more than `max_heaps` of them. Now, they
are abandoned once `max_heaps` more heaps are seen, even if those heaps were
complete. This causes the warnings for incomplete heaps to appear closer to
the time they arrived, and also has some extremely small performance
advantages due to changes in the implementation.

- **backwards-incompatible change**: remove
:py:meth:`~spead2.recv.Stream.set_max_heaps`. It was not previously
documented, so hopefully is not being used. It could not be efficiently
supported with the design changes above.

- Add :py:meth:`spead2.recv.Stream.set_memcpy` to control non-temporal caching
hints.

- Fix C++ version of spead2_bench to actually use the memory pool

- Reduce memory usage in spead2_bench (C++ version)

- Partially fix 40: :py:meth:`~spead2.recv.Stream.set_max_heaps` and
:py:meth:`~spead2.recv.Stream.set_memory_pool` will no longer deadlock if
called on a stream that has already had a reader added and is receiving
data.