recache and uncache the last_change from metadata

The most recent changelog entry is already cached in the metadata as
'last_change' to avoid reading the page changelog on every request.

  - Recache the last change if it is missing and still valid. This is
    needed for when the metadata has been rerendered.

  - Uncaches the last change when it has been invalidated by an external
    edit. Ensures that external edits are not credited to the wrong
    author.

darcs-hash:20061207011005-05dcb-3b1749f6626d07c4cc3de9587c45765423e2e4d9.gz