AutoStore reimplementation

[incipit0]

As discussed with @olynch and @acurrieclark

• Added transaction(), commit() and abort() for AutoStore. Users now have to start and commit transactions whenever they want to read/write the store. Otherwise panic
• Also added the fragment/chunk APIs, for now, only do this when there is no transaction. But it should be possible to do it without this restriction. We can do this once we have a good concurrency control story, which would allow readers to only read up to a particular head.
• Also moved the causal delivery/buffering into coln-store. Now users can send commit bytes to coln-store and if a commit is not ready to be applied, it will be buffered, but the buffer is NOT persisted.