journal: Store offsets to tail entry array objects in chain

journal: Store offsets to tail entry array objects in chain

Previously, we'd iterate an entry array from start to end every time
we added an entry offset to it. To speed up this operation, we cache
the last entry array in the chain and how many items it contains.
This allows the addition of an entry to the chain to be done in
constant time instead of linear time as we don't have to iterate
the entire chain anymore every time we add an entry.