Implement GC for ver4 dict contents.

...l/dictionary/structure/v4/content/bigram_dict_content.cpp

...mpl/dictionary/structure/v4/content/bigram_dict_content.h

...ictionary/structure/v4/content/probability_dict_content.h

...dictionary/structure/v4/content/shortcut_dict_content.cpp

...l/dictionary/structure/v4/content/shortcut_dict_content.h

...ionary/structure/v4/content/sparse_table_dict_content.cpp

...ctionary/structure/v4/content/sparse_table_dict_content.h

...ictionary/structure/v4/ver4_patricia_trie_node_writer.cpp

.../dictionary/structure/v4/ver4_patricia_trie_node_writer.h

...ionary/structure/v4/ver4_patricia_trie_writing_helper.cpp

...ctionary/structure/v4/ver4_patricia_trie_writing_helper.h