Transact now semantics

[burdges]

Users commonly press transact now twice because they want to send a message but the next transaction is a fetch. I believe this behavior is much much worse for anonymity than merely doing a single transact now.

We should probably split the transact now button into "Fetch Now" and "Send Now" variants, maybe using a combo box in the GUI.

Also, we might let users ask pond to clear their outbox in x minutes where x >= 5, so that users can send lots of messages relatively quickly without violating their anonymity too badly.

Ideally, any transaction adjusting behavior should report a warning in the log mentioning the probability of observing that particular behavior from pond's usual chopped off poison distribution, but this requires logging the past behavior.