Make rocksdb the default db (#346)

Author: dangershony

src/Blockcore/Persistence/PersistenceProviderManager.cs

@@ -42,7 +42,7 @@ public string GetDefaultProvider()
                 return null;
             }
 
-            return this.persistenceProviders.ContainsKey("leveldb") ? "leveldb" : this.persistenceProviders.Keys.First();
+            return this.persistenceProviders.ContainsKey("rocksdb") ? "rocksdb" : this.persistenceProviders.Keys.First();
         }
 
         /// <inheritdoc/>
@@ -119,4 +119,4 @@ public void RequirePersistence<TFeature>(IServiceCollection services, string per
             }
         }
     }
-}
+}