Fix default timestamp for portfolio valuation

[lemunozm]

Description

The last_updated field of portfolio valuation is only updating when the valuations are computed. This is ok, nevertheless, if a loan is added to a pool, and modified, it will affect the portfolio valuation without updating the last_updated, which by default will be created with 0 instead of now()

This means that if some entity call nav(), before computing once the portfolio valuation, it will obtain an incorrect time (?). Not sure if this is a possible case or if we always call the update_portfolio_valuation() at least once.

Until I see it, the pool-system is protected when closing the epoch giving an NAVTooOld error, but that means the epoch can not be closed (unless update_porfolio_valuation() is called).

If it is a security issue, I understand we need to deliver a fix soon. In case not, and we can live with it, this will be fixed properly in the Oracle Valuation PR

[lemunozm]

No longer created with 0 by default

[lemunozm]

@mustermeiszer @offerijns It's not clear what should be the last_updated initial value.
In the original pallet-loans implementation, it's initialized in the initialise_pool() extrinsic, but we do not have such a call. That value is defined as "last time when nav is performed for the entire pool", but what if it has never been called? Should we return None instead or the time of first borrowing in the pool?

[mustermeiszer]

I agree it should be the created time.

[lemunozm]

The intent of this commit is to add a fast fix, but if we can wait until the next release where Oracle Valuation fixes this well is better not to merge this PR

[lemunozm]

Close it because the fix is already in #1311