DAL: introduce committee epochs

Fixes #4398 (closed), in that there is a DAL committee per epoch (given by protocol constant, and expressed as a number of blocks), not per level. The committee could be stored in the cache, but this is not done in this MR.

This MR also has a few unrelated minor commits fixing typos or renaming some functions...

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).
• [n/a] 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