QDataStream: handle incomplete reads from QIODevice

This adds a way to resume reading from a stream after a ReadPastEnd error.
This is done by introducing a stream read transaction mechanism that keeps
read data in an internal buffer and rolls it back on failure.

[ChangeLog][QtCore] Added QDataStream startTransaction(),
commitTransaction(), rollbackTransaction(), abortTransaction()
functions to support read transactions.

Task-number: QTBUG-44418
Change-Id: Ibf946e1939a5573c4182fea7e26608947218c2d9
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>