This adds a field for specifying the macaroon root key to use during the
wallet initialization process instead of having lnd generate a random
macaroon root key itself.
This commit adds the --stateless_init flag to all three wallet unlocker
operations. Once you initialize a wallet stateless, you need to set
this flag for every further wallet unlocker operation. Otherwise you
risk non-encrypted macaroon information to leak to the underlying
system.
