* meta: Drop support for "meta link", since supporting this for internal

* meta: Drop support for "meta link", since supporting this for internal
  links required meta to be run during scan, which complicated its data
  storage, since it had to clear data stored during the scan pass to avoid
  duplicating it during the normal preprocessing pass.
* If you used "meta link", you should switch to either "meta openid" (for
  openid delegations), or tags (for internal, invisible links). I assume
  that nobody really used "meta link" for external, non-openid links, since
  the htmlscrubber ate those. (Tell me differently and I'll consider bringing
  back that support.)
* meta: Improved data storage.
* meta: Drop the hackish filter hook that was used to clear
  stored data before preprocessing, this hack was ugly, and broken (cf:
  liw's disappearing openids).
* aggregate: Convert filter hook to a needsbuild hook.