Chain DB: audit and test the iterators/readers for race conditions

The ChainDB iterators and readers will have to read blocks that have been copied from the ImmutableDB and garbage-collected from the VolatileDB. Check whether we deal with this correctly (no race conditions) and test it thoroughly (inspiration: http://hackage.haskell.org/package/dejafu).

Note that in practice there will be a non-trivial overlap between the ImmutableDB and the VolatileDB, so the potential race conditions will probably happen only very rarely (or not at all). Still, we have to test this.