Add a flag to unlock database after each API request

After the first API request, the database was locked as long as the API
server is running. This prevents a user to also use the command-line
client. This commit adds a new flag `-no-lock` that will close the
database after each API request.

Closes #234

cmd/api_serve.go

@@ -46,6 +46,7 @@ Example:
 	}
 
 	cmd.Flag.String("listen", ":8080", "host:port for HTTP listening")
+	cmd.Flag.Bool("no-lock", false, "don't lock the database")
 
 	return cmd