Add site_factor plugin.

Link to build, add site_factor/none as the default (null) plugin.

Connect in to priority_multifactor plugin.

Note that the job_write_lock acquisition in _decay_thread() is promoted
one level to protect both the fair tree use of it alongside the new
site_factor plugin. As fair_tree has been made the default, I felt it was
better to handle it this way, rather than have site_factor reacquire the
locks right after fair_tree dropped them. For sites running without
fair_tree, this does represent a minor drop in potential concurrency.

Bug 5202.