Closes#8069
* Type `ABCIResponses` was just wrapping type `ResponseFinalizeBlock`. This patch removes the former.
* Did some renaming to avoid confusion on the data structure we are working with.
* We also remove any stale ABCIResponses we may have in the state store at the time of pruning
**IMPORTANT**: There is an undesirable side-effect of the unwrapping. An empty `ResponseFinalizeBlock` yields a 0-length proto-buf serialized buffer. This was not the case with `ABCIResponses`. I have added an interim solution, but open for suggestions on more elegant ones.
