Add spender tx/input to tx.output storage.

[evoskuil]

Next to spender height as: [height:4][tx:2][input:2]

This eliminates the spend table for a tiny increase in query cost and small increase in tx table size.

The fetch spend query would be moved into the tx database.

[evoskuil]

This is an optimization that isn't probably necessary, since the block can simply be scanned.