nsqd: add random load balancing for Authd query

[shenhui0509]

In our production environment

1. we have proxies backended with nsqds
2. auth is on
3. proxy uses a connection pool to request on nsqd
   when there is high pub concurrency, proxy will create new connection, thus will query authd. The first authd will be overload, and the latency will increase, result in the risk of avalanche. Add a load balance for authd can ease that.
   on consumer sides, we also have a large number of consumer instances, Load balance for authd is also benefit.
   ready for review

[shenhui0509]

ready for review

[andyxning]

/cc @mreiferson @ploxiln We encounter this in our production environment. PTAL.

[jehiah]

👍 this looks great.

One last thing, can you rebase and squash your commits down to a single commit w/ an appropriate commit message before we merge?

[shenhui0509]

👍 this looks great.

One last thing, can you rebase and squash your commits down to a single commit w/ an appropriate commit message before we merge?

resolved

[andyxning]

@jehiah Thanks for reviewing! Commits are squashed. PTAL.