Side-effect free & lazy multiprocessing context

[m1so]

util._initialize_mp_context/util.mp_context (or basically importing anything from distributed) imports numpy (additional package) as a side-effect, which increases the memory usage by roughly ~100MiB. This change defers the creation of the multiprocessing context from "import time" to "runtime"

Related to PrefectHQ/prefect#5087 (comment)

• Closes #xxxx
• Tests added / passed
• Passes pre-commit run --all-files

[GPUtester]

Can one of the admins verify this patch?

[jakirkham]

ok to test

[m1so]

@jakirkham @mrocklin @martindurant could you please have a look at this? Seems like these are remnants of using "forkserver" multiprocessing context (#687) and side-effects caused by importing modules (#2627).

Reworked and rebased the PR to:

1. disable forkserver related initialization unless explicitly set in config (saving some memory by not import forkserver related objects)
2. checking for package existence instead of importing the modules directly (again saving memory in case the application only imports distributed.Client)

Slack conversation for reference: https://dask.slack.com/archives/C019Q0QSD4J/p1635504992026200

[martindurant]

If I remember this issue clearly, my point was about setting up things like event loops and server components - but I think @jcrist fixed the specific point I found. My memory is a little hazy here. I don't really know the ramifications of preloading modules for forkserver; but only doing so for the case that we actually use the forkserver seems sensible.