[BUGFIX] Cache backend usernames and workspace titles

Add a runtime cache for improved performance of user and
workspace titles in ext:belog. Those view helpers are
called twice per displayed row and should have a shortcut
if the name resolution was done already.

Resolves: #55653
Releases: master, 6.2
Change-Id: I62465faaebc13ba0dfe781350fa6c87cc87f54a6
Reviewed-on: http://review.typo3.org/35525
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>