MFC r209052, r210702, r211095, r211182, r212149: Fix a memory leak and

MFC r209052, r210702, r211095, r211182, r212149: Fix a memory leak and
some potential deadlocks, increase the target limit from 4 to 64, and
numerous other scalability and stability improvements, including.

Submitted by:	Daniel Braniss <danny@cs.huji.ac.il>
Sponsored by:	Dansk Scanning A/S, Data Robotics Inc.

git-svn-id: svn://svn.freebsd.org/base/stable/8@215700 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f