ethdb/pebble: prevent shutdown-panic (#27238)

One difference between pebble and leveldb is that the latter returns error when performing Get on a closed database, the former does a panic. This may be triggered during shutdown (see #27237)

This PR changes the pebble driver so we check that the db is not closed already, for several operations. It also adds tests to the db test-suite, so the previously implicit assumption of "not panic:ing at ops on closed database" is covered by tests.

ethdb/memorydb/memorydb.go

@@ -244,6 +244,9 @@ func (b *batch) Write() error {
 	b.db.lock.Lock()
 	defer b.db.lock.Unlock()
 
+	if b.db.db == nil {
+		return errMemorydbClosed
+	}
 	for _, keyvalue := range b.writes {
 		if keyvalue.delete {
 			delete(b.db.db, string(keyvalue.key))