Prevent database is locked errors

Since there are only SELECT statements, the database can be opened as read-only preventing:

Error: SQLITE_BUSY: database is locked
    at Error (native)

httpd.js

@@ -7,7 +7,7 @@ var moment = require('moment');
 var express = require("express");
 
 var app = express();
-var db = new sqlite3.Database(DB_FILE);
+var db = new sqlite3.Database(DB_FILE, sqlite3.OPEN_READONLY));
 
 var isValidChart = function(s) {
     return /^(net_in|net_out|block_in|block_out|mem)$/.test(s);
@@ -114,4 +114,4 @@ app.get("/rs/all/:chart/:zoom", function(req, res) {
 
 app.listen(8080, function() {
 
-});
+});