Increased robustness

Additions:

• New connectbox_wifi_client_speed_mbit metric

Changes:

• XML responses can be validated against XML schema definitions. Right now, this is only used for the LanUserTable XML.
• Scraping continues if XML parsing fails for an extractor
• Scrape duration and success is now reported for each extractor

Fixes:

• Fixed parsing of the LanUserTable XML to work with the latest firmware