Make DataBroker resource allocation asynchronous

Allocating a TransactionChain and registering a DataTreeChangeListener should be asynchronous.

The first case makes for a weird pattern with error handler, as the handler may fire during allocation. Making allocation asynchronous makes the contract better by separating the allocation failure and normal failure. Normal failure may only occur after the allocation succeeds.

The second case is similar, but also invokes notification. This means that events can only be delivered after the allocation succeeds, hence the user can react to failures separately and it also means that notifications cannot occur as part of register() call.