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.

config_debug.go

@@ -0,0 +1,6 @@
+// +build debug
+
+package main
+
+// DebugBuild signals that this is a debug build.
+const DebugBuild = true