In `bp_blogs_record_existing_blogs()`, switch to using `TRUNCATE` rather than `DELETE` to reset the auto-increment value.

* Move this truncation to after the `empty( $blog_ids )` check, to avoid leaving the tables in a misaligned state. 
* Also truncate `bp_user_blogmeta` to ensure accurate data is recorded.

See #5749.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8585 cdf35c40-ae34-48e0-9cc9-0c9da1808c22