Started to make the buffering more flexible by adding
virtual methods

        virtual sal_uInt32 getHoldCyclesInSeconds() const;
        virtual sal_Int64 estimateUsageInBytes() const;

to class SystemDependentData. This will allow to add more
sensitive buffering/caching.
Also fine-tuned Linux-derived classes actively used for buffering
to be more sensitive when and where to reuse the buffered data

Change-Id: Ifc69c318ade0209aff071d76001869d9f4eeb10d
Reviewed-on: https://gerrit.libreoffice.org/60881
Tested-by: Jenkins
Reviewed-by: Armin Le Grand <Armin.Le.Grand@cib.de>