Hierarchical access to configuration subtrees

[dmlloyd]

Description

As a:

• API user (application developer)
• SPI user (container or runtime developer)

...I need to be able to:

Model the configuration as a hierarchy, and gain access to a hierarchical subsection of the configuration, treating the configuration key as a whole or partial "path" along the hierarchy.

...which enables me to:

• Iterate key-value pairs in the configuration more efficiently (currently the entire configuration must be iterated)
• Reuse configuration-reading code for common configuration subsections without excessive string manipulation

Relates to #334.

[Emily-Jiang]

relates to #240