use UDP for InfluxDB

What does this MR do?

• reenables influxdb metrics
• switches to UDP

How confident are you it won't break things if deployed?

pretty

BEFORE MERGE

@NerdyProjects: update the config.inc.prod.php on the productions server:

define('INFLUX_DSN', 'udp+influxdb://[2a00:1828:2000:664::2]:8089/foodsharing');

Checklist

• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))