Investigate using a mutable type for passphrases and other secrets

[jrick]

Right now, the System.String type is used for passphrases, the wallet seed, etc. This type is immutable and it is impossible (without unsafe) to clear it after it is finished being used. Instead, investigate other methods of storing these secrets (most likely a byte[]), avoiding creating copies of them when creating RPC requests, and zeroing them when finished.

This is not a totally solvable problem (it gets much worse) but a best attempt should be made regardless.