TagCache: More efficient RAM cache utilisation.

Alternatives entries are now stored in the cost-aware manager. Persistently
stored entries are restored with a flag to indicate that no cost calculation
should be performed, and they are subsequently removed when the frame is
destructed (if entries were stored persistently).
Merge branch 'marty/rxml-garb-fixes-5.4' into 5.4

* marty/rxml-garb-fixes-5.4:
  TagCache: More efficient RAM cache utilisation.