Archiving should trigger limited number of concurrent requests

[mattab]

Currently if there are eg. 20 custom segments to generate for a given website, the auto archive command core:archive will trigger 20 concurrent requests to the API.

This can result in some of these requests failing. Instead we should only process at the same time a max count of requests, so that we don't overload the server.

This change will result in better performance of archiving and more reliability when there are many custom segments to pre-process.

[mattab]

In 135624d: Fixes #5035 CliMulti can now trigger a limited number of concurrent requests.
This is set to three concurrent processes in core:archive command.
Refs #4903