Problem

Creating multiple ringct spend transactions in a batch (as part of #970) does not work because of these two similar issues.

1. When creating a transaction, the transaction record has dummy inputs which don't correspond to any actual output, so we don't actually track the pending spends.
2. When verifying that we aren't double-spending any ringct output, we add every keyimage we see to the set to check against. But if we're testing that a transaction could be accepted to the mempool, then committing that transaction, the commit fails (until the next block).

Solution

1. Extract the spent coins' txo records to be marked as pending spend.
2. Don't track the inputs permanently when testing that a transaction could be accepted to mempool. We still track within the transaction.

Tested

On regtest with other changes for #970.