[Engine] `index.fail_on_corruption` is not updateable

The `index.fail_on_corruption` was not updateable via the index settings
API. This commit also fixed the setting prefix to be consistent with other
setting on the engine. Yet, this feature is unreleased so this won't break anything.

Closes #6941
[RESTORE] Fail restore if snapshot is corrupted

today if a snapshot is corrupted the restore operation
never terminates. Yet, if the snapshot is corrupted there
is no way to restore it anyway. If such a snapshot is restored
today the only way to cancle it is to delete the entire index which
might cause dataloss. This commit also fixes an issue in InternalEngine
where a deadlock can occur if a corruption is detected during flush
since the InternalEngine#snapshotIndex aqcuires a topLevel read lock
which prevents closing the engine.

Closes #6938