Move to per-object locks

[jholveck]

Presently, code that requires locks uses the course-grained, global mss.lock. Many of these cases could be moved to per-object locks, if they only require locking over one MSS object's structure, instead of the underlying OS or other global resources. This is suspected to help with #442. It was also mentioned in #443, which started using the global lock to protect against closing an object multiple times.

(Feel free to assign this to me. I just didn't want to forget about it over the holidays!)