Fix jubatus servers to stop mixer before RPC server stopped

fixes #730
Merge pull request #732 from jubatus/fix-730

Fix jubatus servers to stop mixer before RPC server stopped
Add virtual dtor to clustering::storage
Implement mixable_holder::clear_each_mixables
Add clear() to server_base
Modify result type of server_base::clear
Add clear to clustering
Regenerate
Increment revision when clear
Add test for clear
Add clear_mine for clustering::storage