Represents a single transaction broadcast that we are performing. A broadcast occurs after a new transaction is created
(typically by a
Wallet
and needs to be sent to the network. A broadcast can succeed or fail. A success is
defined as seeing the transaction be announced by peers via inv messages, thus indicating their acceptance. A failure
is defined as not reaching acceptance within a timeout period, or getting an explicit error message from peers
indicating that the transaction was not acceptable (this isn't currently implemented in v0.8 of the network protocol
but should be coming in 0.9).