rename mixdepths and mixing levels to reduce confusion

[chris-belcher]

Some people, especially yield-generator users are becoming confused with mixdepths so I think it's better to rename them to something so that the point of them is more obvious. Thinking of going with the name "bucket" or "mixbucket".

[TerminalSausage]

Purse might be a term also, considering bitcoins are often thought of as coins, and coins go in a purse. Perhaps going without the word mix all together. First Coin Purse, Second Coin Purse...