Improve performance of notes cache and notes nullifiers' calculation

[miloszm]

Summary

When testing a user account owning a large number of notes, there are two main performance bottlenecks:

1. adding a note to cache is slow as cache is based on vector while emulating a set behaviour
2. all notes' nullifiers are calculated before each transaction sent, and there is no cache for nullifiers

The above two issues cause multiple minutes of processing for each transaction, when user has a few thousand notes

Possible solution design or implementation

For 1), base the cache implementation on BTreeSet rather than vector
For 2), introduce cache for nullifiers (consider change of the StateClient trait, which currently has the following method:

fn fetch_notes(&self, vk: &ViewKey) -> Result<Vec<EnrichedNote>, Self::Error>;

EnrichedNote type contains Note and block_height only, but it could also contain a nullifier for a given secret key, so that nullifier is also cached. This would save time if the user performs multiple operations with the same large set of notes.

Additional context

This improvement may also apply to other wallets within Dusk, such wallet-cli/wallet-core and a test wallet in Rusk.

[moCello]

In wallet-core there is a function map_owned that creates a map to store the Notes with their nullifiers, maybe that can help?