add initial/experimental support for installing extensions in parallel

[boegel]

WIP because needs enhancements in the test suite + some more testing with R

[boegel]

Tests in latest commit are failing because update_progress_bar gets called without calling start_progress_bar first.

To fix that without jumping through lots of code hoops, I've proposed some trivial changes in #3867...

[branfosj]

My testing has been going well and this looks good.

[boegel]

Now that a test is in place that checks the support for installing extensions in parallel, I think this is ready to merge, in combination with easybuilders/easybuild-easyblocks#2408.

I've tested this extensively on a wide range of R easyconfigs, and I've yet to run into a failing installation because of installing extensions in parallel, so at least for R extensions this seems to work really well.

Quick summary of requirements to also support parallel installation of other types of extensions:

• The run_async implementation should only run a single shell command, which should be run with run_cmd(..., asynchronous=True) via Extension.async_cmd_start. Any other stuff can be run via the existing Extension.prerun or Extension.postrun methods.
• For all extensions involved, the required dependencies must be available via the required_deps property method.

The changes to the EB_toy and Toy_Extension easyblocks in the tests give a good idea of how easyblocks that derive from Extension or ExtensionEasyBlock need to be updated in order to support parallel installation of those type of extensions.

Note that this is marked as an experimental feature and it's opt-in, so it is only used when EasyBuild is configured with both --parallel-extensions-install and --experimental enabled.

[branfosj]

lgtm

[branfosj]

Going in, thanks @boegel!