refactor user_db.
decouple user_dictionary and user_db by making Db a rime class.
refactor db.
refactor db.
add text_db.
extract class UserDbValue.
tsv reader/writer.
fix clang build.
TextFormat.
implement text formats.
dict.yaml: supporting '# no comment' directive.
rewrite UserDictManager::Export().
refactor user_dict_manager.
create snapshots in plain userdb format.
Merge branch 'db'