Complete Log4j2 migration

[m273d15]

With #270/#1064 we migrated the Log4j backend to Log4j2.
Only the framework set-up and initialization had to be migrated
in this step. However, we still use the old API (of log4j) instead
of the log4j2 API.
Migrating the API leads to changes in a myriad of files. Therefore,
we should automate as much as possible.

You can find further information in the logging docs.

[m273d15]

I wrote a small migration script (Disclaimer: This script uses heuristics and the output should be validated). It should migrate all relevant calls, but manual changes are required if:

• A function call is migrated to a closure, but an object is not accessible in the closure context.
• A method is still wrapped in an ifTraceEnabled if-block.

I would recommend avoiding a big bang migration and use the script to migrate packages or components (e.g. core) step-by-step.