Changing config under the guard can cause a deadlock.
The guard holds _read_apply_mutex. The same lock is held by the group0
apply() function. It means that no entry can be applied while the guard
is held and raft apply fiber may be even sleeping waiting for this lock
to be release. Configuration change OTOH waits for the config change
command to be committed before returning, but the way raft is implement
is that commit notifications are triggered from apply fiber which may
be stuck. Deadlock.
Drop and re-take guard around configuration changes.
Fixesscylladb/scylladb#17186
94cd235888