In 0.12, a wallet tx thats not in the mempool does not free up its inputs.  So if a tx spends the same inputs as a conflict tx included in a new block, its inputs are explicitly marked dirty so they will be recalculated as spendable again due to having no non-conflicted spends.  However if the in-block conflict is reorged out, the wallet txs inputs should now be market spent again according to the 0.12 regime, but they are not marked dirty as the tx itself is not notified it is no longer conflicted.

This has the odd effect that the cached value is incorrect and so randomly they will be spendable or not depending on whether they have been recalculated.

This is easy enough to fix by marking all inputs of all txs in SyncTransaction dirty regardless of whether AddToWalletIfInvolvingMe returns true or false.  But I'm not sure the performance impact this will have, so I'm not proposing that change now.

It's a minor issue, but I just thought I'd document that that the cached credit available in wallet txs may not always be correct.

For reference, in 0.11, whenever a tx left the mempool it's inputs were respendable.  So if a tx was removed due to a conflict in a block, any remaining inputs would be spendable again.  If the in-block conflict was later reorged out, the wallet txs inputs would remain spendable unless and until the wallet tx reentered the mempool.

* ReserveDestination is used to reserve an address.
 * It is currently only used inside of CreateTransaction.
 *
 * Instantiating a ReserveDestination does not reserve an address. To do so,
 * GetReservedDestination() needs to be called on the object. Once an address has been
 * reserved, call KeepDestination() on the ReserveDestination object to make sure it is not
 * returned. Call ReturnDestination() to return the address so it can be re-used (for
 * example, if the address was used in a new transaction
 * and that transaction was not completed and needed to be aborted).
 *
 * If an address is reserved and KeepDestination() is not called, then the address will be
 * returned when the ReserveDestination goes out of scope.
 */
class ReserveDestination
{
protected:
    
    const CWallet* const pwallet;
    
    ScriptPubKeyMan* m_spk_man{nullptr};
    OutputType const type;
    
    int64_t nIndex{-1};
    
    CTxDestination address;
    
    bool fInternal{false};
public:
    
    explicit ReserveDestination(CWallet* pwallet, OutputType type)