Pass flush flag through sub chains

[rukai]

process_request_no_return needs some special handling to actually wait for a return on a flush message, which it then ignores.
The wait is purely to ensure the operation completes before shotover shutsdown.
This should be fine performance wise as it only occurs once when the connection is closed and the client isnt waiting on a message response at that point.

[github-actions]

1 benchmark regressed. 0 benchmark improved. Please check the benchmark workflow logs for full details: https://github.com/shotover/shotover-proxy/actions/runs/3672532855

  1 (1.00%) high severe
redis/active_get        time:   [339.59 µs 342.44 µs 347.45 µs]
                        thrpt:  [2.8781 Kelem/s 2.9202 Kelem/s 2.9447 Kelem/s]
                 change:
                        time:   [+22.024% +25.991% +30.086%] (p = 0.00 < 0.05)
                        thrpt:  [-23.128% -20.629% -18.049%]
                        Performance has regressed.