Limit concurrent operations for local / sftp backend

[MichaelEischer]

What does this PR change? What problem does it solve?

Currently, the local and sftp backend are the only ones which do not limit the number of concurrent operations.
This PR adds a Connections parameter for both backends along with the corresponding limits.

Even though Connections sounds unusual for the local backend, I've kept the name for consistency with the other backends.

Was the change previously discussed in an issue or on the forum?

Partially related to #322 and #2679

Checklist

• I have read the contribution guidelines.
• I have enabled maintainer edits.
• [ ] I have added tests for all code changes. The backends already have a good test coverage.
• [ ] I have added documentation for relevant changes (in the manual).
• There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• I have run gofmt on the code in all commits.
• All commit messages are formatted in the same style as the other commits in the repo.
• I'm done! This pull request is ready for review.

[fd0]

LGTM, just one question: why is the number of concurrent operations for the local backend 2, but for the sftp backend 5?

[MichaelEischer]

For the local backend the number is essentially the same as the default from the archiver while reading data. SFTP usually has higher IO latency than a local backend so I've chosen a larger number of connections there. The value 5 is just the number that is also used for all other backends.