Linked key expiration triggers.

#15

Linked expiration works for:
Triggers
Absolute expiration

But not for:
Sliding expiration
Manual removal
Memory pressure removal
Good to know it's an established pattern, but unfortunately I don't have an eye for whether there are any implications or caveats of using it here.

@GrabYourPitchforks can you take a quick look at this call context stuff and see if it's alright by you?