Add experimental "debug attachments throttle <ms>" setting (command line) and ThrottlePer100PrimsRezzed in [Attachments] in config

This is an experimental setting to control cpu spikes when an attachment heavy avatar logs in or avatars with medium attachments lgoin simultaneously.
It inserts a ms sleep specified in terms of attachments prims after each rez when an avatar logs in.
Default is 0 (no throttling).
"debug attachments <level>" changes to "debug attachments log <level>" which controls logging.  A logging level of 1 will show the throttling performed if applicable.
Also adds "debug attachments status" command to show current throttle and debug logging levels.