Spead2

- Drop support for Python 2.
- Drop support for Python 3.4.
- Drop support for trollius.
- Drop support for netmap.
- Avoid creating some cyclic references. These were not memory leaks, but
prevented CPython from freeing objects as soon as it might have.
- Update vendored pybind11 to 2.4.1.

- Add `new_order` argument to :py:meth:`spead2.ItemGroup.update`.
- Improved unit tests.

- Raise :exc:`ValueError` on a dtype that has zero itemsize (37).
- Change exception when dtype has embedded objects from :exc:`TypeError` to
:exc:`ValueError` for consistency
- Remove duplicated socket handle in UDP receiver (67).
- Make `max_poll` argument to :py:class:`spead2.send.UdpIbvStream` actually
have an effect (55).
- Correctly report EOF errors in :cpp:class:`spead2::send::streambuf_stream`.
- Wrap implicitly computed heap cnts to the number of available bits (3).
Previously behaviour was undefined.
- Some header files were not installed by ``make install`` (72).

- Significant performance improvements to send code (in some cases an order of
magnitude improvement).
- Add :option:`!--max-heap` option to :program:`spead2_send` and
:program:`spead2_send.py` to control the depth of the send queue.
- Change the meaning of the :option:`!--heaps` option in :program:`spead2_bench`
and :program:`spead2_bench.py`: it now also controls the depth of the sending
queue.
- Fix a bug in send rate limiting that could allow the target rate to be
exceeded under some conditions.
- Remove :option:`!--threads` option from C++ :program:`spead2_send`, as the new
optimised implementation isn't thread-safe.
- Disable the ``test_numpy_large`` test on macOS, which was causing frequent
failures on TravisCI due to dropped packets.

- Provide manylinux2010 wheels.
- Dynamically link to libibverbs and librdmacm on demand. This allows binaries
(particularly wheels) to support verbs acceleration but still work on systems
without these libraries installed.
- Support for Boost 1.70. Unfortunately Boost 1.70 removes the ability to query
the io_service from a socket, so constructors that take a socket but no
io_service are omitted when compiling with Boost 1.70 or newer.
- Fix some compiler warnings from GCC 8.

- Rework the locking internals of :cpp:class:`spead2::recv::stream` so that
a full ringbuffer doesn't block new readers from being added. This changes
the interfaces between :cpp:class:`spead2::recv::reader` and
:cpp:class:`spead2::recv::stream_base`, but since users generally don't deal
with that interface the major version hasn't been incremented.
- Fix a spurious log message if an in-process receiver is manually stopped.
- Fix an intermittent unit test failure due to timing.