Release retry uses exponential back-off (releases^4).

Timing table:

releases: 0         0 delay,        0 total
releases: 1        1s delay,       1s total
releases: 2       16s delay,      17s total
releases: 3     1m21s delay,    1m38s total
releases: 4     4m16s delay,    5m54s total
releases: 5    10m25s delay,   16m19s total
releases: 6    21m36s delay,   37m55s total
releases: 7     40m1s delay, 1h17m56s total
releases: 8   1h8m16s delay, 2h26m12s total
releases: 9  1h49m21s delay, 4h15m33s total

Derived from:

```go
package main

import (
  "fmt"
  "time"
)

func main() {
  total := time.Duration(0)
  for r := 0; r < 10; r++ {
    d := time.Duration(r * r * r * r) * time.Second
    total += d
    fmt.Printf("releases: %d  %8s delay, %8s total\n", r, d, total)
  }
}
```

-- http://play.golang.org/p/I15lUWoabI

/cc @lox