box: add `confirm_lag` field to `box.info.synchro.queue`

Let's add a lag confirmation field to the limbo, and expose information
about the time the latest successfully confirmed entry waited for quorum.

Part of #9918

@TarantoolBot document
Title: New `age` and `confirm_lag` fields in `box.info.synchro.queue`
Product: Tarantool
Since: 3.2
Root documents: https://www.tarantool.io/ru/doc/latest/reference/reference_lua/box_info/synchro/

The `age` field shows the time that the oldest entry currently present in
the queue has spent waiting for quorum, while the `confirm_lag` field shows
the time that the latest successfully confirmed entry waited for the
quorum to gather.