Remove the abci responses type - prune legacy responses (#8673)

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.

light/store/db/db.go

@@ -13,6 +13,16 @@ import (
 	"github.com/tendermint/tendermint/types"
 )
 
+// key prefixes
+// NB: Before modifying these, cross-check them with those in
+// * internal/store/store.go    [0..4, 13]
+// * internal/state/store.go    [5..8, 14]
+// * internal/evidence/pool.go  [9..10]
+// * light/store/db/db.go       [11..12]
+// TODO(sergio): Move all these to their own package.
+// TODO: what about these (they already collide):
+// * scripts/scmigrate/migrate.go [3]
+// * internal/p2p/peermanager.go  [1]
 const (
 	prefixLightBlock = int64(11)
 	prefixSize       = int64(12)