Add doubled buckets issue

[yngvar-antonsson]

This pull request introduces a new feature to detect and handle doubled buckets in the cluster. The most important changes include adding configuration options, implementing the detection logic, and updating the documentation and tests.

New Feature: Detection of Doubled Buckets

• cartridge.lua: Added configuration options check_doubled_buckets and check_doubled_buckets_period to enable and configure the detection of doubled buckets.
• cartridge/issues.lua: Implemented the logic to check for doubled buckets periodically and raise warnings if any are found. [1] [2]
• cartridge/vshard-utils.lua: Added find_doubled_buckets function to identify doubled buckets in the cluster. [1] [2]

Documentation Updates

• CHANGELOG.rst: Documented the new issue about doubled buckets and how to enable it.
• rst/cartridge_admin.rst: Updated the admin guide to include information about the doubled buckets warning and how to address it.

Testing

• test/integration/vshard_doubled_buckets_test.lua: Added integration tests to verify the detection and handling of doubled buckets.

I didn't forget about

• Tests
• Changelog
• Documentation