channeldb: add migration to properly prune edge update index

In this commit, we introduce a migration to fix some of the recent
issues found w.r.t. the edge update index. The migration attempts to fix
two things:

1) Edge policies include an extra byte at the end due to reading an
extra byte for the node's public key from the serialized node info.

2) Properly prune all stale entries within the edge update index.

As a result of this migration, nodes will have a slightly smaller in
size channeldb. We will also no longer send stale edges to our peers in
response to their gossip queries, which should also fix the fetching
channel announcement for closed channels issue.

channeldb/db.go

@@ -74,6 +74,12 @@ var (
 			number:    5,
 			migration: paymentStatusesMigration,
 		},
+		{
+			// The DB version that properly prunes stale entries
+			// from the edge update index.
+			number:    6,
+			migration: migratePruneEdgeUpdateIndex,
+		},
 	}
 
 	// Big endian is the preferred byte order, due to cursor scans over