MODE-1959 Updated "no-client-transactions" so that they are able to work nested one within the other.

The problem was caused by the fact that internally, in quite a few places including during initial content import, the code starts & commits transactions within which ```writableSessionCache.save()``` calls occur. These calls, start & commit transactions themselves creating in effect a "nesting semantic"

Changed the code in such a way that when ```transactionMode=none``` and these type of nested calls occur, there will be in effect only 1 active transaction at all times.