Refactor Log4j Json Generation

Use jackson directly to serialise the Log4j log events.
Removes the old pattern layout approach.
The new approach provides several callback and customisation
features. They enable users to extend the library within their own
applications. Furthermore, support for custom fields has been
extended to support non-string values for top level fields. Also,
CloudFoundry's VCAP environment variables are no longer added
by default. The user has to opt in those fields

The refactoring gives a factor 3 improvement on the JMH benchmarks
for simple logs and an unchanged performance for request logs:

Benchmark                                          Mode  Cnt       Score        Error  Units
EncodingBenchmarks.minimalRequestRecord           thrpt    5  107274,684 ±  39379,575  ops/s
EncodingBenchmarks.simpleLog                      thrpt    5  504838,974 ±  21313,854  ops/s
EncodingBenchmarks.singleCommonFieldFromMdc       thrpt    5  439796,897 ±  12174,050  ops/s
EncodingBenchmarks.singleCustomFieldFromArgument  thrpt    5  535903,222 ± 109794,213  ops/s

Author: KarstenSchnitter