Update timestamps based on log data

[conorsch]

Right now the @timestamp field stores the time at which the log event was ingested by Logstash. For production systems that's largely OK, but in the event of an outage, or when onboarding a new client with historical log data, the timestamps will be flat-out wrong.

Let's use the date filter to update the timestamps where appropriate. We'll need different date logic per logfile, depending on the built-in time format.

[conorsch]

Handled for existing filters, so closing. The handful of open PRs already have date filters added as well—thanks, @ageis!