dao: migrate nep5 balances with the contract

Fixes #1144. It's quite simple approach, we just update balance info right
upon contract migration. It will slow down migration transactions, but it
takes about 1-2 seconds to Seek through balances at mainnet's 3.8M, so the
approach should still work good enough. The other idea was to make lazy
updates (maintaining contract migration map), but it's more complicated to
implement (and implies that a balance get might also do a write).

There also is a concern about memory usage, it can give a spike of some tens
of megabytes, but that also is considered to be acceptable.

pkg/core/interop_neo.go

@@ -154,6 +154,7 @@ func contractUpdate(ic *interop.Context, v *vm.VM) error {
 		if err := ic.DAO.DeleteContractState(oldHash); err != nil {
 			return fmt.Errorf("failed to update script: %v", err)
 		}
+		ic.DAO.MigrateNEP5Balances(oldHash, newHash)
 	}
 	// if manifest was provided, update the old contract manifest and check associated
 	// storage items if needed
@@ -181,6 +182,7 @@ func contractUpdate(ic *interop.Context, v *vm.VM) error {
 			return fmt.Errorf("failed to update manifest: %v", err)
 		}
 	}
+
 	return nil
 }