Linked key expiration triggers.

#15

Linked expiration works for:
Triggers
Absolute expiration

But not for:
Sliding expiration
Manual removal
Memory pressure removal
If it's always set inside a call to `using(link.FlowContext()) {...}` it should jump out of the context as quickly as possible. Is the concern that there could be a rogue reference that keeps the triggers alive? Then again - the triggers will be added to a cache entry so those will tend to survive GC events also...