Ensure tree-states works with linear_blocks=false

[michaelsproul]

Description

Tree states introduces a new format for storing finalized blocks on disk, which we're planning to make optional via the linear_blocks store config option:

• linear_blocks=false (unstable today): blocks are all stored in the hot DB keyed by root
• linear_blocks=true (tree-states today): finalized blocks are stored compressed in the freezer DB keyed by slot

This issue is about ensuring that Lighthouse can function correctly in tree-states mode with linear_blocks=false. At the moment the configuration option exists, but tree-states ignores it and just assumes blocks are in the freezer DB. We need to ensure that functions like get_blinded_block look-up the correct location.

It may be easier to implement and test this at the same time as the database migration (#4482). Alternatively we could ensure that starting a tree-states node with linear_blocks=false behaves correctly.

For the initial version I don't think we need a way to switch the linear_blocks setting of an existing DB. Users can re-sync if they really want the benefit of that, or we can add such a method later.