bshow next transaction time when sending message

[duy]

• add nextTransactionTime variable to the client to store the next transaction absolute time
• show when next transaction will happen in the "Created new outbox..." message
• remove second decimals from "Next network transaction..." log message

[burdges]

I suspect this patch does not provide the information intended :

Pond's next transaction might not send the message you've just "sent" either because you've multiple messages in the queue already or (I think) even because your next transaction is a fetch not a send.

In principle, you could reasonably display this time if (a) no messages are currently in the queue and (b) the next transaction would be a send. If (a) fails, maybe you'd want to say how many messages are ahead of it in the queue. If only (b) fails, then maybe you'd want to say "approximately" nextTransactionTime + 5min.

I donno if @agl had other concerns with this patch besides those.