Phs/rename tier to relaton and uniqify file names

This emphasizes how central the idea is about to become.  Since they,
not tables, are the largest typed construct, application authors will
spend most of their time working with them (and their components.)

Ideally this word would be "table", with the HBase notion of table
being relegated to a (critically important) implementation detail.
That would cause too much confusion though, so "relation" it is.

I've also renamed the schema classes by appending the suffix `Schema`.
This leaves `Relation` open as a name, which we'll use shortly to represent
the object Askari applications wrap an HBase client with to perform typed
crud operations.