fix usedsize bug and free logic
Merge branch 'appendonly' of https://github.com/ctriposs/bigcache into appendonly

Conflicts:
	src/main/java/com/ctriposs/quickcache/SimpleCache.java
	src/main/java/com/ctriposs/quickcache/storage/StorageBlock.java
	src/test/java/com/ctriposs/simplecache/SimpleCacheStressTest.java