Add context hierarchy to adiak metadata

[mplegendre]

Add hierarchy to adiak metadata. For example,

adiak::push_context(“mylib”)
adiak::value("version", ...)
adiak::value("foo", ...)
adiak::value(…)
adiak::pop_context()

Produces names like “mylib::version”, “mylib::foo”, …

Some metadata should probably always go to the root context. For example, 'user' or 'launchday' does not make sense in a lower-level hierarchy.

Also, we have been asked to provide tool callbacks when context is popped.

[baallan]

Probably want a tool callback when context is pushed as well, especially if adiak doesn't add timestamp to every value.
Also want a tool callback at adiak-cleanup for any unpopped context, perhaps with a flag set telling the tool that it's an exit-generated pop and not a user-generated pop.

[baallan]

I suggest for hierarchical stuff, we produce dot qualified instead of :: qualified string keys. this would be in line with being more json-like in the choice of [] and {} syntax.

[baallan]

In the use-case of independent libraries/sub-applications, we should think about allowing the developer calling push/pop to optionally specify that they want their information to be independent/top level instead of a nested context child. One way to do this without adding functions would be to document that if a context name begins with "$." that it (less the $.) is a top level name.