Limited cache lifetime for mrna-protein links

Rework the caching of transcript<->protein accession number links. If no
link is reported by the NCBI, this is now also cached. For these type of
negative links and the normal links, separate cache lifetimes can be
defined.


git-svn-id: https://humgenprojects.lumc.nl/svn/mutalyzer/trunk@698 eb6bd6ab-9ccd-42b9-aceb-e2899b4a52f1