Make FetchStreamFromURL::aBuffer a pointer to prevent the crash that
happens because of it.

Add ExternalDataMapper::StopImport() method.

Add a few member functions in CSVFetchThread that help in sync the
line fetch and writing.

Introduce a WriteToDoc() method that iterates over the the range to
apply whatever value or string needs to be applied.

Change-Id: I9baa053fa6d91bdf758a9b035888af50a40674eb
Reviewed-on: https://gerrit.libreoffice.org/32964Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>