Improve semantics of implicit txn management for async transactions

There is a usability issue for transaction implicit management API.

For async transaction tx.commit must be called after all async ops are enlisted to the transaction. This is a responsibility of a user in case of explicit management.

But, for implicit management, tx.commit must be called automatically. This is not generally not possible for async flows, because by the end of a closure some operations are not yet started.

The example:

runInTransactionAsync(tx ->

)

We can fix this by introducing async context for implicit management and require a user to return last completion stage in the chain, like:

<T> CompletableFuture<T> runInTransactionAsync(Function<Transaction, CompletableFuture<T>> clo);