[LST-415] fix rebalance

[billythedummy]

update_yield should be performed wrt the old_sol_value stored in the RebalanceRecord instead of the diminished one in PoolState.

Other

• update doc regarding when we should release_yield
  • apply changes based on this update
• rename lst_sync_sol_val -> lst_ssv_uy
• delete tokenkeg test fixture, dynamically override missing fixture accounts as needed instead
• add more asserts to swap test
• other refactors