Always store kernel excess in db

[lehnberg]

As proposed by @jaspervdm during dev meeting. Full discussion on gitter: https://gitter.im/grin_community/dev?at=5d9373c9eb1eff63d65a27bf

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Description

• Currently, the kernel excess of a tx is stored only if the user is the sender and there is no change output.
• This should change so that the kernel excess is always stored for every transaction inserted into the db, regardless if the user is a sender or receiver.

Motivation

• It can't be calculated after the fact, so if the user needs it sometime later for whatever reason, they are out of luck;
• Users can use it to quickly find a particular tx on block explorers;
• Exchanges/pools/payment providers can use it to check on-chain status without relying on the grin-wallet. They can also communicate it to their users as something that is analogous to an on-chain txid

Note

• Not every tx will always have a excess associated to it (for example it won't be recoverable when restoring from seed).
• While introducing this change, it MUST be ensured that the code that's selectively doing the kernel lookup is not triggered. It's currently triggered based on the presence of a stored kernel excess.