__dask_distributed_pack__(): client argument

[madsbk]

This PR implements the new client argument of Layer.__dask_distributed_pack__() introduced by dask/dask#6850

[jrbourbeau]

Hmm we're getting a msgpack error when running distributed/tests/test_client.py::test_profile_server (see https://github.com/dask/distributed/runs/1414687711#step:8:117):

   File "msgpack/_packer.pyx", line 286, in msgpack._cmsgpack.Packer.pack
  File "msgpack/_packer.pyx", line 292, in msgpack._cmsgpack.Packer.pack
  File "msgpack/_packer.pyx", line 289, in msgpack._cmsgpack.Packer.pack
  File "msgpack/_packer.pyx", line 225, in msgpack._cmsgpack.Packer._pack
  File "msgpack/_packer.pyx", line 225, in msgpack._cmsgpack.Packer._pack
  File "msgpack/_packer.pyx", line 225, in msgpack._cmsgpack.Packer._pack
  [Previous line repeated 508 more times]
  File "msgpack/_packer.pyx", line 223, in msgpack._cmsgpack.Packer._pack
  File "msgpack/_packer.pyx", line 157, in msgpack._cmsgpack.Packer._pack
ValueError: recursion limit exceeded.

Though this seems unrelated to the changes in this PR. I've pushed an empty commit to trigger CI and see if the failure persists. I don't recall seeing it before.

[jrbourbeau]

Alright, it looks like the test_profile_server failure is unrelated to the changes in this PR. It doesn't consistently fail on CI and I've not been able to reproduce it locally. I opened up #4251 in case this failure pops up on other PRs.