Proto/Liquidity Baking: decrease escape hatch threshold and increase sunset level

Context

Fixes #2074 (closed).

• Increases the sunset level by twenty voting periods.

2_244_609 (previous sunset) 
+ 20 * 40_960 (blocks_per_voting_period) 
= 3_063_809 (new sunset)

• Lowers the threshold for triggering the escape hatch from 50% to 33%.

Manually testing the MR

Similar to !2765 (merged):

• The escape hatch can be invoked through a per_block_votes JSON file (that can be used for changing constants in a future upgrade), e.g. tezos-baker-alpha --votefile "per_block_votes.json" where per_block_votes.json contains {"liquidity_baking_escape_vote": true}.

• Unit tests can be run with dune exec src/proto_alpha/lib_protocol/test/main.exe -- test "^liquidity baking$".

Checklist

• (Not applicable) Document the interface of any function added or modified (see the coding guidelines)
• (Not applicable) Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR