Handle etcd cluster connectivity issues with watch.
Handle etcd cluster connectivity issues with watch.

Current State: When the whole etcd cluster goes down, all the watches would return
  with error ErrWatchStoppedand. With 1. we will resubscribe, but this call gets
  stuck as it is unable to create a new session with any of the etcd nodes. Most
  of the pxctl volume commands will simply stuck doing nothing and trying to
  reach etcd.

  Fix: Add a timeout while creating sessions in etcdv3.Watch. With this change
  subscribe will not get stuck but instead if there is a timeout in creating a
  session we will just return ErrWatchStopped. After 5 retries of subscribe we
  will exit and restart.

portworx/kvdb