Convert pre-Magnesium to Magnesium snapshots/journal entries

Magnesium streaming format explicitly does not support BigInteger values, simply because they are invalid values – they are superseded by Uint64s. A similar situation exists in mapping of uint8,16,32 types, where they are no longer widened to java.lang types.

When we are recovering a pre-Magnesium stream, we need to take this change into account and instruct NormalizedNodePruner to convert these values.