stop hitting git when fetching post metadata

stop hitting git when fetching post metadata

Inspired by a patch from Aleix Conchillo Flaqué.  Thanks, Aleix!

* tekuti/index.scm (index-specs):
* tekuti/post.scm (reindex-posts, reindex-posts-by-date): Change to have
  the "posts" index be a hash table mapping key -> post.
  "posts-by-date" is a list of keys, from newest to oldest.
  (post-from-git): This is what post-from-key was.
  (post-from-key): New function, pulls out a post from the posts hash
  table.
  (munge-post): Use post-from-git.
  (delete-post): Take the post directly.
  (latest-posts): New awesome helper for fetching the lastest N posts
  that match some predicates.

* tekuti/page-helpers.scm (find-posts-matching): Adapt to post-from-key
  change.
  (published-posts): Remove, replaced with latest-posts.

* tekuti/page.scm (page-admin, page-admin-posts, page-admin-post)
  (page-archives, page-feed-atom): Use the new latest-posts helper.
  (page-admin-delete-post, page-admin-delete-comment, page-show-post)
  (page-new-comment, page-show-tag): Adapt to post-from-key change.

* tekuti/tags.scm (compute-related-posts, compute-related-tags): Adapt
  to post-from-key change.
  (reindex-tags): Use latest-posts.