Protect against wedged managers

[aaronlehmann]

If a manager gets stuck with the store lock held (for example, as in #1651), this does not affect heartbeats, and there won't be a new leader election. It's a very bad failure mode because you end up with a stuck leader that doesn't get replaced. We should consider ways to reduce the impact of this situation.

Perhaps the leader should stop sending heartbeats if the store lock has been held for too long?

Or perhaps other managers should periodically poll a noop RPC endpoint on the leader that takes the store lock, and start a leader election if it times out?

cc @aluzzardi @LK4D4