rpcserver+lnrpc+lncli: add AbandonChannel rpc call

Using AbandonChannel, a channel can be abandoned. This means
removing all state without any on-chain or off-chain action.
A close summary is the only thing that is stored in the db after
abandoning.

A specific close type Abandoned is added. Abandoned channels
can be retrieved via the ClosedChannels RPC.

cmd/lncli/main.go

@@ -264,6 +264,7 @@ func main() {
 		openChannelCommand,
 		closeChannelCommand,
 		closeAllChannelsCommand,
+		abandonChannelCommand,
 		listPeersCommand,
 		walletBalanceCommand,
 		channelBalanceCommand,