Implement a JSON file terminus to mirror YAML.

For a bunch of local caching and retrieval, Puppet uses a YAML backed local
file terminus to store, retrieve, and search content. This is great, but the
YAML encoder is super-slow and painfully odd across Ruby versions.

Our PSON support is much more reliable, as well as substantially faster, and
is pretty much as capable. There isn't much reason, other than migration
cost, to prefer YAML to PSON encoding of data transfer documents.

Having a JSON terminus allows the user to configure, manually, the use of JSON
everywhere that we previously used YAML. While this doesn't bring the
benefits out of the box, it allows users to select this while we flush out any
lingering bugs left in the support code and system.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>