lnd: move seed generation outside wallet to allow for user supplied entropy
lnd: move seed generation outside wallet to allow for user supplied entropy

In this commit, we extend the initial wallet creation set up case with
the goal of giving the user the ability to restore a prior wallet from
seed, or obtain the mnemonic for a newly generated wallet.

As the WalletUnlocker has been extended to allow passing a user source
of entropy, if this is detected, then we’ll use BIP39 to covert it into
an HD wallet seed. Otherwise, we’ll generate our own entropy, then
convert that into the wallet seed.

In order to make this change, we’ll now manually create the default
wallet ourselves. In the case that the user didn’t provide their own
seed, we’ll send the seed we generated back to the user. This will allow
frontends to display the newly generated seed to the end user.

aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd