This commit makes merging of batches with empty batches efficient.
Previously this operation would be handled like a regular merge, by
allocating a new batch and merging the input batches into it. However,
if one of the batches is empty, there is no need to allocate a new
batch, we can simply extend the bounds of the existing non-empty batch
accordingly.

The implementation of this optimization is complicated by the fact that
there are `Batch` implementations for `Rc<Batch>` and
`Abomonated<Batch>`. Both of these only provide an immutable reference
to the underlying batch, making it impossible to adjust the batch
description to extend the batch's bounds. This commit solves this by
introducing new `RcBatch` and `AbomonatedBatch` wrappers that, in
addition to the `Rc`/`Abomonated` batch, hold their own `Description`s
that override the inner batch's description.

This commit makes sure completed merges are logged also for merges that
used the empty batch optimization.

Also, if we log the dropping of a merged batch, we shouldn't forget to
also log the merge event, so consumers don't become confused about the
whereabouts of the input batches.