Rework retry policy

[robx]

This is a follow-up to #40.

• Maximal delay

  The retry policy stops retrying once we hit a delay of 6 seconds. This was likely intended to be 60 seconds. The effect of this is that the configurable total number of retries is effectively limited to 6 (with delays 0.1, 0.2, 0.4, 0.8, 1.6, 3.2 seconds).

• Use fullJitterBackoff, as per this comment Fix inaccurate retry policy comment, and clarify current behaviour #40 (comment).