core: Move replay + error correction logic to commit log

Makes `CommitLog` a read-only handle and introduces `CommitLogMut`,
allowing to append. This better encapsulates the error detection and
trimming logic, and enforces it to be executed before being able to
mutate the log.

crates/core/src/database_instance_context.rs

@@ -97,12 +97,24 @@ impl DatabaseInstanceContext {
 
     /// The number of bytes on disk occupied by the [MessageLog].
     pub fn message_log_size_on_disk(&self) -> Result<u64, DBError> {
-        self.relational_db.commit_log().message_log_size_on_disk()
+        let size = self
+            .relational_db
+            .commit_log()
+            .map(|commit_log| commit_log.message_log_size_on_disk())
+            .transpose()?;
+
+        Ok(size.unwrap_or_default())
     }
 
     /// The number of bytes on disk occupied by the [ObjectDB].
     pub fn object_db_size_on_disk(&self) -> Result<u64, DBError> {
-        self.relational_db.commit_log().object_db_size_on_disk()
+        let size = self
+            .relational_db
+            .commit_log()
+            .map(|commit_log| commit_log.object_db_size_on_disk())
+            .transpose()?;
+
+        Ok(size.unwrap_or_default())
     }
 
     /// The size of the log file.