Logic:
- If sending a transaction, assign its timestamp to the current time.
- If receiving a transaction outside a block, assign its timestamp to the current time.
- If receiving a block with a future timestamp, assign all its (not already known) transactions' timestamps to the current time.
- If receiving a block with a past timestamp, before the most recent known transaction (that we care about), assign all its (not already known) transactions' timestamps to the same timestamp as that most-recent-known transaction.
- If receiving a block with a past timestamp, but after the most recent known transaction, assign all its (not already known) transactions' timestamps to the block time.
For backward compatibility, new accounting data is stored after a \0 in the comment string.
This way, old versions and third-party software should load and store them, but all actual use (listtransactions, for example) ignores it.
