Introduce functions for getting the sent-date of a specific private message, without putting it through time-since calculations. This allows for plugins and themes to perform custom calculations on message output like "37d" or "14m" or "15s", without needing to touch the global directly.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8692 cdf35c40-ae34-48e0-9cc9-0c9da1808c22