Add support for prev_value param to _update_meta() functions

This parameter, inherited from update_metadata(), allows you to limit your
updates only to rows where the meta_value matches your specified prev_value.

See #5400

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7885 cdf35c40-ae34-48e0-9cc9-0c9da1808c22