core, ethdb: reuse database batches (#15989)

* leveldb: Update leveldb to 211f780 (poolfix)

* core, ethdb: reuse database batches

ethdb/memory_database.go

@@ -123,3 +123,8 @@ func (b *memBatch) Write() error {
 func (b *memBatch) ValueSize() int {
 	return b.size
 }
+
+func (b *memBatch) Reset() {
+	b.writes = b.writes[:0]
+	b.size = 0
+}