Each abci begin block call, the historical info will get stored and pruned
according to the HistoricalEntries parameter.
If the HistoricalEntries parameter is 0, then the BeginBlock performs a no-op.
Otherwise, the latest historical info is stored under the key historicalInfoKey|height, while any entries older than height - HistoricalEntries is deleted.
In most cases, this results in a single entry being pruned per block.
However, if the parameter HistoricalEntries has changed to a lower value there will be multiple entries in the store that must be pruned.