crucible: fix further instances of copy-after-unlock bug

Before:

	unique_lock<mutex> lock(some_mutex);
	// run lock.~unique_lock() because return
	// return reference to unprotected heap
	return foo[bar];

After:

	unique_lock<mutex> lock(some_mutex);
	// make copy of object on heap protected by mutex lock
	auto tmp_copy = foo[bar];
	// run lock.~unique_lock() because return
	// pass locally allocated object to copy constructor
	return tmp_copy;

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

include/crucible/lockset.h

@@ -161,7 +161,10 @@ namespace crucible {
 	LockSet<T>::copy()
 	{
 		unique_lock<mutex> lock(m_mutex);
-		return m_set;
+		// Make temporary copy of set while protected by mutex
+		auto rv = m_set;
+		// Return temporary copy after releasing lock
+		return rv;
 	}
 
 	template <class T>