(PDB-454) - Updating the catalogs endpoint to return environments

This changeset does several things. It changes the catalog endpoint's results
to use the new version 4 wire format. It also lays down some ground work for
a canonical catalog format. Currently there are several different internal representations
of a catalog in the code. This attempts to unify it with our new definition of
wire format versions and enforce it with Schema.
Fix for the terminus to stringify version per spec, added defensive conversion code in the back end.

Changed export to use version 3 explictly, rather than the latest version (will update as part of another ticket)
Changed to covert catalog version to a string for wire formats v1-v3. Also a
minor doc update for the terminus.
Merge pull request #908 from senior/ticket/master/pdb-454-environment-querying

(PDB-454) - Updating the catalogs endpoint to return environments