etcdserver, auth: not cache a flag of auth status

[mitake]

This commit removes a flag that indicates auth is enabled or disabled
because it doesn't have an invalidation mechanism.

Fixes #3601 and #3964

[mitake]

cc @xiang90 @gzliudan @densetsunozero

[xiang90]

LGTM. I am not sure about the performance impact. (after this change, even if you do not enable auth, there is an overhead). Ideally, we should run a benchmark test.

[mitake]

OK, I'll do benchmarking. Please wait for a while.

[xiang90]

@mitake Thanks a lot.

[mitake]

I run my benchmark of 10,000 key creation (https://github.com/mitake/etcd-things/blob/master/prioritize-leader-bench/bench.go , without prioritizing leader) on the master branch and this PR. There was no significant differenct:

• master

real    1m32.631s
user    0m1.112s
sys     0m0.453s

• this PR

real    1m32.812s
user    0m1.162s
sys     0m0.375s

I think this PR doesn't introduce significant overhead because current auth flag checking doesn't require quorum, just local lookup. I combined this PR and #3994 , that requires quorum in the auth flag checking. Actually, the overhead was easily observed.

• this PR + quorum auth

real    2m15.012s
user    0m0.943s
sys     0m0.639s

Anyway, if we integrate auth into etcdserver, consistent and low overhead auth mechanism will be possible.

[mitake]

@gyuho I think this commit should be backported to the bugfix releases for reviving the auth mechanism.

[xiang90]

@mitake I agree. Github was down while we were back-porting commits...

[mitake]

@xiang90 ah, I see :)