blockstore: OpenReadOnly shouldn't need to parse the entire index into memory in some cases

[mvdan]

Right now we always read and unmarshal an existing whole index into memory.

Luckily, IndexSorted is designed to allow using directly as a mmapped []byte from disk. If OpenReadOnly is called with a carv2 file that has a sorted index, we should use the index straight from the mmap.

cc @masih @willscott

[masih]

Relates to #95