Cache - cache an incoming asset.

DecrementTTL - issue was expired so decrease lock count.

Delete - remove an asset from the cache.

Exists - check if an item confirmed in storage handle.

Finalise - stop all background handlers.

Get - get packed asset data from cache (nil if not present).

IncrementTTL - lock ass as used by one issue.

Initialise - setup the asset cache.

SetVerified - mark a cached asset being verified.