Make save transaction-friendly

We need to make `save()` (and potentially `sync()`) transactions-friendly.

By default we don't want `save()` functionality to produce partial results, instead it is intended as an atomic all-or-nothing operation. Since it's not always possible to pack the necessary operations into a single query we may use a transaction internally to make `save()` atomic. We don't have nested transactions in Gel, so this behaviour of `save()` is not compatible with being nested inside another transaction.

We need to automatically detect whether we're in a transaction already and use savepoints when needed for `save()` to allow rolling back changes in case of save errors without breaking the outer transaction overall.