Make $.fn.updateTooltipAccessKeys() less expensive

Make $.fn.updateTooltipAccessKeys() less expensive

On page.ready on desktop skins, we call $.fn.updateTooltipAccessKeys(), which
updates about 20 elements (depending on the page and skin). Each invocation
parser the 'brackets' message, which ends up being quite expensive. We
previously had some caching to mitigate the cost, but it was removed in
I955ee61f6.

We can re-introduce caching without re-introducing the bug that I955ee61f6
sought to mitigate by making the astCache property unique to each parser
instance, as suggested in T54042#545717, and by ensuring the message function
is cached.

Change-Id: Ia1b42c70d8fc6ce03c8708f03c2b835942c4ead3