Hash table shrink

 After the pull request #90, we can do the expanding with only a item lock. So it is real easy to complete the shrink of the hash table.  When the items in the hash table is less than 2/3 of the bucket, cause a hash table shrink. The process of the shrink is almost the same as the expanding. I add a variance 'hashpower_delta' to indicate expanding or shrink, that is all the different between expanding and shrink.