Introduces bp_*_user_last_activity() functions

User last activity data is currently stored as usermeta, with BuddyPress
making multiple calls directly to the usermeta table to get that data. These
new functions provide a central, standardized way to update and fetch
last_activity information. This makes it easier for plugins to update user
activity programatically in a future-compatible way.

Having this set of functions will facilitate any changes associated with #5128,
whose goal is to move last_activity data out of usermeta altogether.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7334 cdf35c40-ae34-48e0-9cc9-0c9da1808c22