[#203] Fix bugs of new ID allocation in store entities (#204)

### What changes were proposed in this pull request?
- Allow nested transactions in RocksdbKvBackend
- Introduce transaction in method `put` of `KvEntityStore` 

### Why are the changes needed?
Currently, new ID allocation logic depends on transactions, So we have
to add transactions in the method `put` that will possibly trigger new
ID allocation.

Fix: #203 

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

Add some test code in existing UT `testCreateKvEntityStore`

core/src/main/java/com/datastrato/graviton/storage/kv/KvEntityStore.java

@@ -91,10 +91,14 @@ public boolean exists(NameIdentifier ident, EntityType entityType) throws IOExce
   @Override
   public <E extends Entity & HasIdentifier> void put(E e, boolean overwritten)
       throws IOException, EntityAlreadyExistsException {
-    // Simple implementation, just use the entity's identifier as the key
-    byte[] key = entityKeyEncoder.encode(EntityIdentifer.of(e.nameIdentifier(), e.type()), true);
-    byte[] value = serDe.serialize(e);
-    backend.put(key, value, overwritten);
+    executeInTransaction(
+        () -> {
+          byte[] key =
+              entityKeyEncoder.encode(EntityIdentifer.of(e.nameIdentifier(), e.type()), true);
+          byte[] value = serDe.serialize(e);
+          backend.put(key, value, overwritten);
+          return null;
+        });
   }
 
   @Override

core/src/main/java/com/datastrato/graviton/storage/kv/RocksDBKvBackend.java

@@ -204,6 +204,11 @@ public void close() throws IOException {
   @Override
   public <R, E extends Exception> R executeInTransaction(Executable<R, E> executable)
       throws E, IOException {
+    // Already in tx, directly execute it without creating a new tx
+    if (TX_LOCAL.get() != null) {
+      return executable.execute();
+    }
+
     Transaction tx = db.beginTransaction(new WriteOptions());
     LOGGER.info("Starting transaction: {}", tx);
     TX_LOCAL.set(tx);

core/src/test/java/com/datastrato/graviton/storage/kv/TestEntityStorageBackend.java

@@ -189,6 +189,16 @@ public void testCreateKvEntityStore() {
           NoSuchEntityException.class,
           () -> store.get(metalake.nameIdentifier(), EntityType.METALAKE, BaseMetalake.class));
 
+      // Add new updateMetaLake.
+      // 'updatedMetalake2' is a new name, which will trigger id allocation
+      BaseMetalake updatedMetalake2 =
+          new BaseMetalake.Builder()
+              .withId(metalake.getId())
+              .withName("updatedMetalake2")
+              .withAuditInfo(auditInfo)
+              .withVersion(SchemaVersion.V_0_1)
+              .build();
+      store.put(updatedMetalake2);
     } catch (Exception e) {
       Assertions.fail(e.getMessage());
     }