1. Implement a pass-through for garbage collection on the idstore.
2. Fix the `lotus-shed state-prune` command.
NOTE: the new performance of running a full prune will be significantly
less as this version doesn't batch. However, it should now actually
_work_ and most users will be using the splitstore anyways.
This paves the way for better object lifetime management.
Concretely, it makes it possible to:
- have different stores backing chain and state data.
- having the same datastore library, but using different parameters.
- attach different caching layers/policies to each class of data, e.g.
sizing caches differently.
- specifying different retention policies for chain and state data.
This separation is important because:
- access patterns/frequency of chain and state data are different.
- state is derivable from chain, so one could never expunge the chain
store, and only retain state objects reachable from the last finality
in the state store.
Motivation:
* Run lotus with the race detector enabled (primary motivation).
* Allow multiple lotus nodes in a process (not a high priority).
Previously, the journal was shared between all lotus instances, but it was
initialized for every new node. This caused safety problems in tests (at a
minimum).
This patch explicitly passes the journal to all services that need it.
