Refactor ChainedReader to produce contained readers itself

The work on DBZ-175 revealed some shortcomings with the ChainedReader class: all the contained readers need to be known and added upfront. That's problematic as typically the exact configuration of reader (N + 1) only is known after reader N has completed its work (e.g. to set a starting offset). This leads to lots of references between readers e.g. in ParallelSnapshotReader and the reconciling reader. This all could be simplified if the chained reader would produce the contained readers itself lazily, i.e. if reader N is completed, it would set up reader N + 1 based on the state of N.