bp_core_current_time() not being used for all timestamps

[DJPaul]

In trunk, not all of the timestamps are being generated by the bp_core_current_time() wrapper function: some of the 'last_activity' timestamps, xprofile data 'last_updated' and bbPress' bb_insert_topic() and bp_forums_insert_post().

bp_core_current_time() is BP's wrapper function around current_time() because old versions of BuddyPress had been very inconsistent about whether a GMT timestamp was provided (or not).

As some of these last_updated/_activity timestamps are re-generated every few minutes, and others are not exposed in the BP-Default theme, this appears to have had minimal impact so far.

[djpaul]

(In [3455]) Use bp_core_current_time() for all timestamps. Fixes #2750.