Inline xml writing into Event class

[rakow]

The current XML event writer is very inefficient. For every event that needs to be written:

• A map is created
• Attributes put into the map and often converted to a new string object
• Every attribute will be XML encoded
  • Which is not needed for primitives
  • Creates a new string in case the attribute needs to be encoded
• Multiple calls to a BufferedWriter are made
  • Which is a synchronized data structure and not very efficient with a lot of invocations

This PR moves the serialization code to the events itself and makes use of a StringBuilder.
Event classes are responsible to write their xml representation via writeAsXML(StringBuilder out) directly to the builder. Using this approach, not a single new object needs to be allocated for writing an event.

Events who don't overwrite this method will use the old behavior. The resulting output should be 100% identical.
For now writeAsXML was implemented for the most often used events and more may be added in further PRs.

In a quick test on a realistic but not terrible large scenario, the mobsim was roughly speed up by 10%.