Secondary Instance should pass read_only=true

[jay-zhuang]

Quote @ajkr https://github.com/facebook/rocksdb/pull/8171/files#r612195411:

I think DBImpl should be passed read_only=true. That's a new flag introduced in #8142 to prevent SanitizeOptions() from creating an info logger when none is provided (i.e., info_log == nullptr). I'd guess in a secondary DB, a SanitizeOptions()-created info logger could conflict with the primary DB info logger. It looks like OpenAsSecondary() tries to avoid opening the DB with option info_log == nullptr -- except when L616 is executed.

[jay-zhuang]

Seems minor issue, included it in #8171.