`meta.DB.Open(readOnly)` does not move metabase in RO mode

[carpawell]

Expected Behavior

Metabase is asked to open bbolt in RO, and I expect the internal state to be RO too.

Current Behavior

Bbolt is opened in RO, however, metabase accepts write operations and returns bbolt's error.

Possible Solution

Open sets mode to RO (additional test for it is also good).

Context

Firstly found in https://github.com/nspcc-dev/neofs-node/pull/2884.