Track and report wallet transaction clones

Adds a "walletconflicts" array to transaction info; if
a wallet transaction is mutated, the alternate transaction id
or ids are reported there (usually the array will be empty).

Metadata from the original transaction is copied to the mutant,
so the transaction time and "from" account of the mutant are
reported correctly.
Merge pull request #3671 from gavinandresen/txn_conflicts

Report transaction conflicts, and tentative account balance fix