lnd: integrate leader elector to lnd main

This commit also changes the order of DB init to be run after the RPC
server is up. This will allow us to later add an RPC endpoint to be used
to query leadership status.

sample-lnd.conf

@@ -1066,6 +1066,20 @@ litecoin.node=ltcd
 ; If non zero, LND will use this as peer port for the embedded etcd instance.
 ; db.etcd.embedded_peer_port=1235
 
+[cluster]
+; Enables leader election if set.
+; cluster.enable-leader-election=true
+
+; Leader elector to use. Valid values: "etcd" (default).
+; cluster.leader-elector=etcd
+
+; Election key prefix when using etcd leader elector. Defaults to "/leader/".
+; cluster.etcd-election-prefix=/leader/
+
+; Identifier for this node inside the cluster (used in leader election).
+; Defaults to the hostname.
+; cluster.id=example.com
+
 [bolt]
 ; If true, prevents the database from syncing its freelist to disk. 
 ; db.bolt.nofreelistsync=1