Push row key types to row_key_type on tiers.

Tiers within a table are now a list.  This has the advantage of not
having to invoke a YAML parser (again) to validate (or otherwise visit
a tier.)

Similar changes for qualifier are coming.
Push a qualifier's name down into the qualifier_type key.

Qualifiers in a family now form a list.  They are still required to
have unique qualifier_types.  To keep things unambiguous, the existing
type attribute has been renamed to value_type.

Now all of the yaml-ish structure in our schema is exposed after a
single parse.
Migrations are schema clauses dedicated to describing row migrations.
Give each tier a set of migrations.