It's possible that a user might not want the Tor private key to sit on the disk in plaintext (it is a private key after all). So this commit adds a new flag to encrypt the Tor private key on disk using the wallet's seed. When the --tor.encryptkey flag is used, LND will still write the Tor key to the same file, however it will now be encrypted intead of plaintext. This essentially uses the same method to encrypt the Tor private key as is used to encrypt the Static Channel Backup file.
