Add jitter to HA deduping heartbeats

[gouthamve]

Right now there is a thundering herd every 15s (updateTimeout) where all distributors will be CASing the ring. With 25+ distributors, I think this puts undue load on etcd. I think we can do better by adding jitter to the heartbeats.

[adnxn]

@gouthamve will take a look at this one

[gouthamve]

Closed this in #1748