Store/allow tx metadata in all undo records

Previously, transaction metadata (height, coinbase or not, and before
the previous commit also nVersion) was only stored for undo records
that correspond to the last output of a transaction being spent.

This only saves 2 bytes per undo record. Change this to storing this
information for every undo record, and stop complaining for having it
in non-last output spends. This means that undo dat written with
this patch won't be readable by older versions anymore.

src/undo.h

@@ -12,8 +12,7 @@
 
 /** Undo information for a CTxIn
  *
- *  Contains the prevout's CTxOut being spent, and if this was the
- *  last output of the affected transaction, its metadata as well
+ *  Contains the prevout's CTxOut being spent, and its metadata as well
  *  (coinbase or not, height). Earlier versions also stored the transaction
  *  version.
  */