Fix the block last finalized pointer returned by GetBlockInfo.

[td202]

Purpose

Fixes #1002.

Changes

• Change the implementation of lastFinalizedOf to correctly get an ancestor of a block that is considered finalized when examining the chain just to the given block. In particular, this requires two blocks to be in the same epoch, as well as in consecutive rounds.
• Change the implementation of getBlockInfo to always use the result provided by lastFinalizedOf, instead of using the block itself when it is finalized.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.