Add `userWritesAllowed()` check to disagg oplog cap maintainer thread startup to mirror [analogous check](https://github.com/10gen/mongo/blob/master/src/mongo/db/storage/oplog_cap_maintainer_thread.cpp#L159) in ASC.

On disagg this is mainly to prevent oplog truncation from running during magic restore mode. `userWritesAllowed()` should always be false when `storageGlobalParams.magicRestore` is true.