Improve TX broadcast

[aakselrod]

It's worthwhile to try to improve transaction broadcast for both additional privacy (trickling to peers instead of sending to all) and propagation monitoring (which isn't possible to do with the current setup as we turn off transaction inv messages). We should investigate how to improve this without giving up privacy by turning the transaction inv messages on and off when we send a transaction; this may involve additional mechanisms for watching unconfirmed transactions such as a more detailed inv as suggested here.

[nopara73]

A Dandelion implementation may be the cleanest, if not the easiest way.

Otherwise, if Tor support is added, then transactions could be propagated on isolating streams or with NEWNYM requests.