Changed the note scanning algorithm to not require additional context.

[batconjurer]

Describe your changes

Simplified the note scanning algorithm so that it does not require a merge operation when new keys are introduced into the wallet. Instead of having two paths in the fetching code: one for when the viewing keys are unchanged but there are new shielded transactions from the network, and the other for when new keys are introduced and transactions must be rescanned from the beginning. Instead of doing that, we now have a single path that starts scanning transactions from the most out of date viewing key going forward to the present. This change increases the set of valid states that a ShieldedContext can have and allows its (recoverable) state to be saved more frequently.

Indicate on which release or other PRs this topic is based on

Namada v0.30.2

Checklist before merging to draft

• [x] I have added a changelog
• [x] Git history is in acceptable state