Blogs: Mirror a blog's various comment options to BP's blogmeta.

This commit mirrors the following blog options to BP's blogmeta:
- 'close_comments_for_old_posts'
- 'close_comment_days_old'
- 'thread_comments_depth'

These options will be used later in the new bp_blogs_comments_open()
function to determine whether a blog post's activity item should be
closed from commenting.  This is designed to prevent multiple calls
to switch_to_blog().

See #5130

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8189 cdf35c40-ae34-48e0-9cc9-0c9da1808c22