Add Consul KV store based on atomic transactions

[timvaillancourt]

Related issue: #1273

Description

This PR introduces a new, optional KV store for performing atomic Consul KV updates. This KV store hopes to address a race condition when performing updates of several key-values, context: #1273

This new KV store updates many KVPairs by:

1. Getting the current values from Consul in a single transaction
2. Skipping KVPairs that are already up-to-date (same as go/kv/consul.go)
3. Skipping KVPairs in the local cache (same as go/kv/consul.go)
4. Updating all KVPairs that require an update in a single transaction

A new Consul library was required to add this support, the official consul api library. Unfortunately this new library has a tonne of dependencies. At first govendor fetch github.com/hashicorp/consul/api added 1500+ files to the vendor/ dir. After manually removing various dependencies I was able to get it compile with just 500~ additional files (👎)

It seems the bulk of the dependencies come from github.com/hashicorp/go-hclog, which requires github.com/mattn/go-isatty which requires the huge library golang.org/x/sys/unix (about 300 of the 500 files) 😕. We could shave off a few more files by deleting _test.go files, but I suspect this might confuse govendor. Any ideas to reduce deps further appreciated!

Lastly, I ran govendor remove +unused to drop more deps, which explains the few unrelated files deleted from vendor/

cc @shlomi-noach

[shlomi-noach]

Looks good! This being opt-in and backwards compatible is great. Thank you for adding a documentation section.

[shlomi-noach]

Closes #1273