15s block times

This is similar to !4163 (closed), except that it reduces block times to 15s instead of 20s.

Updates to the documentation will be added in later commits or in another MR.

Checklist

• [n/a] Document the interface of any function added or modified (see the coding guidelines)
• [n/a] Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR