Remove zebra_state::Request::FindBlockHeaders

[hdevalence]

Is your feature request related to a problem? Please describe.

The zebra_state::Request::FindBlockHeaders request variant duplicates functionality of Request::FindBlockHashes, but adds more work, blocking the state queue from processing other requests. Once the list of block hashes has been atomically computed, there's no need to atomically fetch all of the blocks to obtain their headers, so there is no correctness problem with moving that phase of the work out of the state service. This simplifies the state implementation.

Describe the solution you'd like

1. Change FindBlockHashes to include a max_len: usize, passed directly to self.find_chain_hashes;

2. Replace the existing use of FindBlockHeaders in the Inbound service by a FindBlockHashes, followed by a sequence of Block requests;

3. Remove FindBlockHeaders from the state API.

Describe alternatives you've considered

Leaving the code as-is. This prevents a clean separation of concerns and prevents us from composing middleware in front of the state service.

[teor2345]

This is obsolete - with the new database and state API, we should:

• move this request to the ReadStateService
• only query the finalized state block headers column family