Abstract zcash code into a plugin

[pavlenex]

In 1.13.0 we're abstracting payment methods, allowing for greater flexibility and also it will finally allow us to free up the codebase from altcoin code once and for all.

Once all the refactoring is done on the payment method side, we will remove zcash and abstract it's code into a plugin as an example how it can and should be done for any other current and future payment method.