Add support for expiration to the context resolver.

[tobz]

Context

Currently, ContextResolver lacks support for expiring contexts. In many cases, as older, unused contexts stop getting used, we should be able to reclaim the memory they're consuming, This is obviously important in the general sense -- we shouldn't hold on to memory we don't need anymore -- but is doubly important as it also means we're not able to reclaim space in the string interner if those interned strings are kept around forever.

[tobz]

Closed by #252.