Multi-recipients

[jaytaph]

Implement the multi-recipient setup (see wiki https://github.com/bitmaelum/bitmaelum-suite/wiki/Multiple-recipients-(CC-and-BCC) and https://github.com/bitmaelum/bitmaelum-suite/wiki/Proposal-multi-recipients).

• delivery pool
  configurable pool of workers that will deliver a block either remotely or locally.
• proof-of-work pool
  this pool will either create a proof-of-work ticket, or returns quickly with a "in-progress" status.
  it should cache tickets so we can later fetch the ticket for uploading other blocks as well.
• setup receiving side that will push blocks to the queue
• update ticket system on client and server side to add extra info (header, blocks etc)

func createPool() {
  for i:=0; i!=25; i++ {
    go poolWorker()
  }
}
func poolWorker() {
  for {
    data <- queueChannel
    ticket := ticketPool.getTicket(data.Header)
    if ticket == nil {
        // requeue data for later when the ticket is finished
        queueChannel <- data
        continue
    }
    deliver(data.Header, data.Block) // or catalog, or attachment)
}