channeldb: add NoMigration mode

In case the channeldb package is used as a library in external tools, it
can be useful to allow read-only access to a DB. This allows such a
tool to access a DB even if not all migrations were executed, which can
be useful for recovery purposes.
To make it possible to even start the DB with a read-only backend, we
need to disable the automatic migration step.

routing/pathfind_test.go

@@ -175,7 +175,7 @@ func makeTestGraph(useCache bool) (*channeldb.ChannelGraph, kvdb.Backend,
 	graph, err := channeldb.NewChannelGraph(
 		backend, opts.RejectCacheSize, opts.ChannelCacheSize,
 		opts.BatchCommitInterval, opts.PreAllocCacheNumNodes,
-		useCache,
+		useCache, false,
 	)
 	if err != nil {
 		cleanUp()