Spead2

The :doc:`ibverbs <py-ibverbs>` acceleration has been substantially modified to use a
newer version of rdma-core. It will no longer compile against versions of
MLNX-OFED prior to 5.0. Compiled code (such as Python wheels) will still run
against old versions of MLNX-OFED, but extension features such as multi-packet
receive queues and packet timestamps will not work. It is recommended that if
you are using ibverbs acceleration with older MLNX-OFED drivers that you stick
with spead2 2.x until you're able to upgrade the drivers and spead2
simultaneously.

Other changes:

- Support hardware send rate limiting when using ibverbs.
- Discover libibverbs and pcap using pkg-config where possible.
- Make :program:`configure` print out the configuration that will be compiled.
- Update the Python wheels to use manylinux2014. This uses a newer compiler
(potentially giving better performance) and supports :c:func:`sendmmsg`.
- Add wheels for Python 3.9.
- A number of deprecated functions have been removed.
- Avoid ibverbs code creating a send queue for receiver or vice versa.
- Rename ``slave`` option to :program:`spead2_bench` to ``agent``.

- Make verbs acceleration work when run against MLNX OFED 5.x, including with
Python wheels. Note that it will not use multi-packet receive queues, so
receive performance may still be better on MLNX OFED 4.9.

- Update pybind to 2.5.0.
- Fix compilation against latest rdma-core.
- Some documentation cleanup.

- Support unicast receive with ibverbs acceleration (including in
:program:`mcdump`).
- Fix :program:`spead2_recv` listening only on loopback when given just a port
number.
- Support unicast addresses in a few APIs that previously only accepted
multicast addresses; in most cases the unicast address must match the
interface address.
- Add missing ``<map>`` include to ``<spead2/recv_heap.h>``.
- Show the values of immediate items in :program:`spead2_recv`.
- Fix occasional crash when using thread pool with more than one thread
together with ibverbs.
- Fix bug in mcdump causing it to hang if the arguments couldn't be parsed
(only happened when capturing to file).
- Fix :program:`spead2_recv` reporting statistics that may miss out the last
batch of packets.

- Log warnings on some internal errors (that hopefully never happen).
- Include wheels for Python 3.8.
- Build debug symbols for binary wheels (in a separate tarball on Github).

- Fix race condition in TCP receiver (78).
- Update vendored pybind11 to 2.4.2.