Rework the ConfigurationPropertySources and related adapter classes to
help with performance. The ConfigurationPropertySources class now only
monitors for updates when `.attach` is used. The `.get` methods now
return the adapted version, but no longer checks to see if sources have
been added or removed on each call.

This commit also fixes a few caching issues and makes both the
`PropertyMapper` implementations true static singletons.

See gh-9000