Checkpoint hangs for readonly DB

> Note: Please use Issues only for bug reports. For questions, discussions, feature requests, etc. post to dev group: https://groups.google.com/forum/#!forum/rocksdb or https://www.facebook.com/groups/rocksdb.dev

### Expected behavior

A read only or secondary DB is checkpoint-able

### Actual behavior

The process hangs. I suspect it's due to this change: https://github.com/facebook/rocksdb/pull/8968#discussion_r935932468

### Steps to reproduce the behavior

Open A DB as secondary or readonly, try to make a checkpoint out of it.