Implement assoc_vector
Use assoc_vector in inverted_index_storage
Use vector instead of unordered_map
Make get_max_id method to key_manager and use it in inverted_index_storage

size method of key_manager is not equal to max id in it because a user may remove some ids in key_manager.
When a user remove an id, it causes segmentation fault.
Use fixed_size_heap instead of sort
Fix msgpack format for assoc_vector
Use const_cast in find method
Only compare keys in find method and operator []
Add random test for assoc_vector
Implement clustering::clear
Fix clustering::clear to clear all data
add linear_normalization_filter
add gaussian normalizer
add sigmoid normalizer
some minor change
Fix gmm_clustering_method's ctor
Clear clustering_method's state when empty list is given
Clear clustering_method when clear() is called
Do clustering when model is loaded
Fix storage::clear to dispatch REVISION_CHANGE after clear