[sheepdog] [sheepdog/sheepdog] 1c0f71: lib: exponential backoff in request retry

[Hitoshi Mitake]

  Branch: refs/heads/exp-backoff
  Home:   https://github.com/sheepdog/sheepdog
  Commit: 1c0f71c9fa78c499a9927e6f7558ba7d104055f8
      https://github.com/sheepdog/sheepdog/commit/1c0f71c9fa78c499a9927e6f7558ba7d104055f8
  Author: Hitoshi Mitake <mitake.hitoshi at lab.ntt.co.jp>
  Date:   2017-02-15 (Wed, 15 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>