Conflicts:
VisualStudio/Arango Front End Library/Arango Front End Library.vcxproj
arangod/Actions/RestActionHandler.cpp
arangod/RestServer/ArangoServer.cpp
lib/ApplicationServer/ApplicationServer.cpp
this allows clients to send a content-id for each batch part
the server will return the content-id in each part response so the client can more easily map requests and responses
the content-id will not be validated or checked for uniqueness by the server. it is up to the client to send sensible content-ids to the server
currently, the server will not store the content-ids anywhere. they are just used within one batch request and then "forgotten"