#1 delDocByUnid - maintain "deleted" flag in document

[Th Kn]

Problem:
if a document is deleted, the list of available document
unids doesn't contain the old unid of the deleted doc.
Chances are that this unid is reassigned to a new
document.

Furthermore I need a function to mark a document
deleted and add a DUMP DOCUMENTs function later if
nesseccary.

Possible Solution:
add a hidden field "_deleted" to a document
when "delDocByUnid" is called rather than deleting the
doc itself. Then add methods like "doc->isDeleted()"
and "db->isDocDeletedByUnid()" to check if docs are
marked for deletion.