Partially convert Rdb_cf_options class to use std::string_view

[laurynas-biveinis]

• Partially convert Rdb_cf_options methods to take std::string_view parameters. Specifically, do not convert the ones that are passed as keys to m_name_map, which is a std::unordered_map<string, string>. This can be converted in C++20, where lookups through a different key type are supported.
• Convert Rdb_cf_manager::is_cf_name_reverse and create_cf_flags_if_needed methods to take std::string_view arguments, update callers.
• Convert pointer arguments for the touched methods to const reference and reference ones, where possible, remove redundant nullptr-checking asserts. Remove redundant const from pass-by-value arguments, add [[nodiscard]].
• Update ER_CF_DIFFERENT to print an std::string_view parameter.

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[facebook-github-bot]

This pull request has been merged in facebook/mysql-5.6@68638be03ecd66ba4caf3a9196bdf575afab1327.