lnwallet: fetch the root key during startup rather than on creation
lnwallet: fetch the root key during startup rather than on creation

This commit modifies the initialization logic of the LightningWallet to
fetch the root key during startup rather than during creation. We make
this change in order to give enough time for the underlying
WalletController to properly boot up before we ask it to do any work.

aakselrod/lnd,LightningNetwork/lnd,AndrewSamokhvalov/lnd,LightningNetwork/lnd,AndrewSamokhvalov/lnd,ddustin/lnd,aakselrod/lnd,ddustin/lnd