Convert DomainCreateFlow to use generic tm() methods (#1026)

Various necessary changes included as part of this:

- Make ForeignKeyIndex completely generic. Previously, only the load()
method that took a DateTime as input could use SQL, and the cached flow
was particular to Objectify Keys. Now, the cached flow and the
non-cached flow can use the same (ish) piece of code to load / create
the relevant index objects before filtering or modifying them as
necessary.
- EntityChanges should use VKeys
- FlowUtils should persist entity changes using tm(), however not all
object types are storable in SQL.
- Filling out PollMessage fields with the proper object type when
loading from SQL
- Changing a few tm() calls to ofyTm() calls when using objectify. This
is because creating a read-only transaction in SQL is quite a footgun at
the moment, because it makes the entire transaction you're in (if you
were already in one) a read-only transaction.

core/src/test/java/google/registry/flows/ResourceFlowTestCase.java

@@ -121,6 +121,10 @@ public abstract class ResourceFlowTestCase<F extends Flow, R extends EppResource
    * Confirms that an EppResourceIndex entity exists in Datastore for a given resource.
    */
   protected static <T extends EppResource> void assertEppResourceIndexEntityFor(final T resource) {
+    if (!tm().isOfy()) {
+      // Indices aren't explicitly stored as objects in SQL
+      return;
+    }
     ImmutableList<EppResourceIndex> indices =
         Streams.stream(
                 ofy()

core/src/test/java/google/registry/persistence/transaction/TransactionManagerTest.java

@@ -137,7 +137,7 @@ public class TransactionManagerTest {
     assertThat(persisted).isEqualTo(theEntity);
   }
 
-  @TestOfyAndSql
+  @TestOfyOnly // read-only not implemented in SQL yet
   void transactNewReadOnly_throwsWhenWritingEntity() {
     assertEntityNotExist(theEntity);
     assertThrows(