Support for Etcd v3

[chungers]

This PR adds etcd3 as the backend for leadership detection and storage.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "etcd" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354184288
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[codecov]

Codecov Report

Merging #436 into master will decrease coverage by 1.42%.
The diff coverage is 20.6%.

@@            Coverage Diff             @@
##           master     #436      +/-   ##
==========================================
- Coverage   64.52%   63.09%   -1.43%     
==========================================
  Files          82       86       +4     
  Lines        5133     5298     +165     
==========================================
+ Hits         3312     3343      +31     
- Misses       1446     1576     +130     
- Partials      375      379       +4

Impacted Files	Coverage Δ
pkg/leader/etcd/v3/etcd.go	0% <0%> (ø)
pkg/store/swarm/swarm.go	67.27% <0%> (-5.28%)	⬇️
pkg/store/etcd/v3/etcd.go	0% <0%> (ø)
pkg/util/etcd/v3/etcd.go	28.57% <28.57%> (ø)
pkg/util/exec/exec.go	37.33% <37.33%> (ø)
pkg/broker/client/sse.go	61.62% <0%> (-3.49%)	⬇️

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