FIX: Use the same mode for chinese search when indexing and querying. (PR #14780)

The 白名单 term becomes 名单 白名单 after it is processed by cppjieba in :query mode. However, 白名单 is not tokenized as such by cppjieba when it appears in a string of text. Therefore, this may lead to failed matches as the search data generated while indexing may not contain all of the terms generated by :query mode. We’ve decided to maintain parity for now such that both indexing and querying uses the same :mix mode. This may lead to less accurate search but our plan is to properly support CJK search in the future.