Introduce field $TaskID

[rsoika]

• replace $ProcessID with the new standard field $taskID

this is to become more conform with the concept of a event driven workflow engine.

Provide getter/setter methods and log warning in case of old usage - but we still support old filednames.

IMPORTANT
To avoid wrong search results we need to place a warning into the lucene-seach method if the $TaskID is used in query.

BUT
We must support both variants for a longer time!

See Issue #384