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box: drop iproto connections before client fibers on shutdown

Currently we have such order of shutting down: client fibers, then
iproto subsystem and then fiber pool (details can be found in e513d4c).
The problem is iproto produces new fibers in `tx_fiber_pool` when
processing new requests so when Tarantool is shutting down client
fiber, a new one can appear and it won't be handled. Moreover, there is
no point in handling such fibers because one can appear even after all
client fibers are finished.

Also, we cannot just stop `tx_fiber_pool` so that it won't produce new
workers while shutting down client fibers because it's also needed for
vinyl. But since only iproto can produce new client (non-system)
long-living fibers when shutting down, let's simply drop all iproto
connections before finishing client fibers. From user's point of view,
one thing will be changed - he will receive `Peer closed` error message
instead of `fiber is cancelled` for hanging requests. I don't think it's
bad because the new message better reflects that Tarantool is being
stopped and that the error is not retriable.

By the way, we already disallow accepting new iproto connections right
before shutdown (`iproto_on_shutdown_f`) but the problem is connected
client can keep sending requests. And that's why our replication tests
became flaky - if shutdown happens between JOIN and SUBSCRIBE, fiber
pool produces new client fiber after we cancelled all such fibers.

Follow-up #9722
Closes #10313

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