Replace usage of PostgreSQL hstore type

Switch back to the standard schema format now that the incomptible hstore type
is not present.
Make SingleValue.update thread-safe

Use a @@class_var (shared with subclasses) rather than an @instance_var (unique to each class)
even though there will be no race condition between classes/types, so each subclass does
not need to initialize @mutex.
Switch database backend from PostgreSQL to SQLite
Fix SQLite3 boolean check

This prevents everyone from being counted as retired when the field IS NULL.
Cover the boolen check in the next_suitable query.
Install database_cleaner now that we tests that involve the database.
Merge branch 'sqlite'