Port to use /hashicorp/hcat library

https://github.com/hashicorp/hcat (hashicat) is a library encompassing all the dependency monitoring functionality that envconsul uses from consul-template. It is meant to be the common library used by all projects that currently use consul-template as a library.

Switching to it will enable all the various project that use consul-template now to benefit from improved API and features, eliminating a lot of duplicate work.