Problem

Different keyspaces may require different floor LSNs in vectored
delta layer visits. This patch adds support for such cases.

Summary of Changes

* Rework layer visit collection. Each layer type has a separate
visit type which is aware of the requirements. For delta layers
we track the floor LSN of keyspaces and merge only when that's
matching. Other layer types do not have this requirement so they
merge everything.
* Thread the new visit types into the `get_values_reconstruct_data`
calls. For delta layers, the code was adapted such that it may
merge reads across keyspaces with different LSN floor requirements.
* Tweak the fringe update code in `get_vectored_reconstruct_data_timeline`
to handle different cont LSNs for different keyspaces. In practice,
we will only update the fringe from one keyspace, since this "keyspace
split" only happens when an image layer overlaps a delta layer (and
image layers always complete all their keys).
* Update tests with the new interfaces

`ValuesReconstructState::consume_done_keys` may only be called once
after a layer visit. The code in the previous commit called it for
each keyspace, resulting in keys not being marked done in the split
keyspace by floor LSN scenario.