Finish migration to event.duration for 4 Filebeat modules

[webmat]

This PR is built on top of #10188 and should not be merged before it.

Please only review commits from Jan 23rd or later.

Filebeat modules affected

• elasticsearch.slowlog
• haproxy.log
• mysql.slowlog
• postgresql.log

Introduced in this PR

• All pipelines use a temp value and a common Ingest Node painless processor, to reduce script compilations.
  • MySQL uses temp field but not shared code, as it requires a tweak to avoid float imprecision problems
• Old field names removed from fields definitions. Since the scale changes by a factor or 1 million or 1 billion, we can't make them aliases with a straight face.
• Each of these migrated fields gets a special entry in ecs-migration.yml, which mentions the scale difference between old field and event.duration, and marks them as alias: false.

[webmat]

@ycombinator Tweak to event.duration handling for elasticsearch.slowlog

[ycombinator]

ES slowlog fileset change LGTM. Since it's a breaking change – the elasticsearch.slowlog.took_millis field is no longer being indexed — worth documenting in the breaking changes section of the CHANGELOG?

[ruflin]

LGTM

[webmat]

Adding to a changelog like this: kosher?

[webmat]

Failing libbeat test seems unrelated to me. No field changes here could cause a problem, AFAIK.