Perform delayed coalescing prior to purging.

Rather than purging uncoalesced extents, perform just enough incremental
coalescing to purge only fully coalesced extents.  In the absence of
cached extent reuse, the immediate versus delayed incremental purging
algorithms result in the same purge order.

This resolves #655.

include/jemalloc/internal/extent_structs.h

@@ -116,8 +116,11 @@ struct extents_s {
 	/* All stored extents must be in the same state. */
 	extent_state_t		state;
 
-	/* If true, try to coalesce during extent deallocation. */
-	bool			try_coalesce;
+	/*
+	 * If true, delay coalescing until eviction; otherwise coalesce during
+	 * deallocation.
+	 */
+	bool			delay_coalesce;
 };
 
 #endif /* JEMALLOC_INTERNAL_EXTENT_STRUCTS_H */