Force preStart to poll if primary hasn't written snapshot

[tgross]

In #22 (comment) @tgross wrote:

The leader election process happens in the health check, and we can't restore the snapshot once mysqld is running so it has to happen in the preStart. We might be able to change this by forcing a replica to poll/wait during preStart if a primary has been elected but there's no backup yet.

In #28 (comment) @tgross wrote:

Maybe the replicas came up before the snapshot was done being written but once we check Manta/Consul everything is in place? That might be an area to improve

So clearly it looks like we're sometimes going to jump the gun on launching replicas before the primary has fully "settled" by writing its snapshot. Fixing this would make this more feasible for automated deployment.

cc @misterbisson

[tgross]

Released in https://github.com/autopilotpattern/mysql/releases/tag/5.6r2.3.0