Update librustzcash to 6.1

[rob1997]

What

Update current reference for librustzcash to 6.1

Why

6.1 includes new security, performance and functionality updates

How

Either by referencing 6.1 then refactoring current implementations around it or by forking 6.1 and refactoring it to be compatible with current implementations.

Acceptance Criteria

6.1 referenced and used in webZjs

Estimation

@sqhell feel free to add your own estimation

Dependencies

References
Architecture
Slack Thread
Problem
Upgrade Grant
@sqhell feel free to add your own additional references

[sqhell]

I have been working on forking this, and also fixing bugs around allowing users to sync to the birthday of their original wallet.

Estimation:

I have done most of the work, and still finding the odd bug while manually testing, I should have a PR ready early next week for review.

[sqhell]

ChainSafe/librustzcash-nu61#2

#153