[sheepdog] [sheepdog/sheepdog] 66952d: lib: exponential backoff in request retry

[Hitoshi Mitake]

  Branch: refs/heads/exp-backoff
  Home:   https://github.com/sheepdog/sheepdog
  Commit: 66952d39c1563bb755621eb6334bcc26e8d00be0
      https://github.com/sheepdog/sheepdog/commit/66952d39c1563bb755621eb6334bcc26e8d00be0
  Author: Hitoshi Mitake <mitake.hitoshi at lab.ntt.co.jp>
  Date:   2017-02-16 (Thu, 16 Feb 2017)

  Changed paths:
    M dog/Makefile.am
    M include/net.h
    M lib/net.c
    M shepherd/Makefile.am

  Log Message:
  -----------
  lib: exponential backoff in request retry

Current sheepdog resend a request after timeout immediately. It isn't
good for avoiding high load spikes: requests that come in a short time
interval will be resent at the almost same time. This commit adds a
mechanism of exponential backoff [1] for distributing the retry
pressure.

[1] https://www.awsarchitectureblog.com/2015/03/backoff.html

Signed-off-by: Hitoshi Mitake <mitake.hitoshi at lab.ntt.co.jp>