jobqueue: Add job_type to PSR logging context

jobqueue: Add job_type to PSR logging context

The mediawiki.runJobs errors are collected in Logstash but the whole job
description and errors are a single field message. That is challenging
to split logs per job type, get the longest running jobs ...

That can be worked around on the log receiving side by parsing
MediaWiki messages eg https://gerrit.wikimedia.org/r/#/c/312504/

Bryan Davis suggested a better long term solution is to use the PSR3
logger with structured log messages.

Culprit: 'type' is a reverved word. Hence prefix all context variables
with 'job_'.

Bug: T146469
Change-Id: Ib6a771c7d3f83bd75b2994bfab9bbebfd1f5aa6c