[WIP] Feature/parallel download

[nkoenig]

Not looking for a review yet.

🎉 New feature

Closes #1260

Summary

This replaces #1669.

The PR changes the Server load to:

1. Disable Fuel downloads.
2. Parse the SDF file/string.
3. Remove all models, actors, and lights from each world.
4. Create the worlds (runners).
5. Enable Fuel downloads and force simulation to be paused. User cannot unpause.
6. Start a thread that parses the SDF file again. This time models are downloaded.
7. Add the models back into the worlds.
8. Allow the pause state in simulation to change.

The above will be followed even if nothing needs to be downloaded.

Test it

All of the tests should pass, and you should be able to clear your fuel cache and load world like tunnel.sdf

Checklist

• [] Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[nkoenig]

@osrf-jenkins run tests please.