lnwallet: switch over to using btcwallet's new Loader type
lnwallet: switch over to using btcwallet's new Loader type

Allows us to remove a lot of duplicated code pertaining to wallet
setup. We also gain hooks into the wallet to trigger callbacks once the
wallet is created/opened.

BitfuryLightning/lnd,bryanvu/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd,ddustin/lnd,aakselrod/lnd,AndrewSamokhvalov/lnd,LightningNetwork/lnd,BitfuryLightning/lnd,bryanvu/lnd,LightningNetwork/lnd,ddustin/lnd