Upgrade to etcd 3.4

[palourde]

Background

We should consider upgrading etcd to v3.4. There's a couple of significant improvements that we could probably benefit from. Additionally, etcd now recommends 3.3.18+, and 3.4.2+ as minimal versions for production.

One important thing to notice is that etcd 3.4 switched to go modules and is now known as the go.etcd.io/etcd/v3 module, which signifies we will need to update the import path in our codebase.

Acceptance Criteria

• sensu-go with etcd 3.4 is benchmarked to make sure it doesn't introduce any performance regression
• The new 3.4 features are reviewed and spikes and/or feature requests are open if there's anything significant we should explore/implement.

Definition of Done

• sensu-backend w/ etcd 3.4 is benchmarked
• sensu-backend w/ etcd 3.4 passes the automated QA

[palourde]

We might have to wait for 3.5: etcd-io/etcd#11154

[nikkictl]

For multiple reasons (described in #3760 (comment)), we must wait for etcd v3.5 to update etcd, bolt, and go.

[portertech]

@eric to comment.

[eric]

I have no comment 😬

[portertech]

@eric This made my day 😆

Will the real @echlebek please stand up.

[nikkictl]

We can track the v3.5 milestone here: https://github.com/etcd-io/etcd/milestone/37

[amdprophet]

Implemented in #4229.