feat: Implement delete for the storage engine (#777)

* docs: Fix incorrect comment of Vector::only_null

* feat: Add delete to WriteRequest and WriteBatch

* feat: Filter deleted rows

* fix: Fix panic after reopening engine

This is detected by adding a reopen step to the delete test for region.

* fix: Fix OpType::min_type()

* test: Add delete absent key test

* chore: Address CR comments

src/datatypes/src/vectors.rs

@@ -111,7 +111,7 @@ pub trait Vector: Send + Sync + Serializable + Debug + VectorOp {
     /// Returns whether row is null.
     fn is_null(&self, row: usize) -> bool;
 
-    /// If the only value vector can contain is NULL.
+    /// If the vector only contains NULL.
     fn only_null(&self) -> bool {
         self.null_count() == self.len()
     }