Transactions should unblock downstream transactions sooner

[dimitribouniol]

This can happen as soon as the transaction closure returns, but before writes. Another transaction can safely rely on this state being the common ground at this point, as any write failures will cause failures for this transaction as well. Note the transaction itself shouldn't return early, as that would violate the durability promise. However, this optimization should allow more writes to take place across many requests, for instance, increasing overall throughput.