Protect against nil job in new allocation

Job revert

Bumping up Vagrantfile's nomad version

Update constraint.html.md

Fix Distinct Constraint header

Refactor Consul Syncer into new ServiceClient

Fixes #2218

Support for TLSSkipVerify in other checks coming soon!

Previous implementation assumed all struct fields were included in
service and check IDs. Service IDs never include port labels and check
IDs *optionally* include port labels, so lots of things had to change.

Added a really big test to exercise this.

...and still protect against leaking agent entries in Consul on
shutdown.