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applier: don't send a stale ACK on fiber cancellation

Before the patch the applier writer fiber waited for an ACK request on
`writer_cond` and, as soon as `fiber_cond_wait_timeout()` returned, sent
the next ACK without checking why it woke up.

The wait can also return because the fiber was cancelled. In that case
we shouldn't send one last ACK: `coio_write_xrow()` may not yield if the
socket is writable right away, so a stale ACK could slip out before the
fiber stops. Let's check for fiber cancellation after the wait and break
out instead.

Whenever `writer_cond` is signalled `has_acks_to_send` is set to true.
Let's add that assertion in order not to break it one more time, it can
lead to nasty bugs. Though, the assertion only makes sense for peers >=
1.7.7 (where the wait is infinite), old versions wake up on timeout and
don't have `has_acks_to_send` set.

Follows up #12795

NO_DOC=internal
NO_TEST=can't provoke the cancellation race from Lua, hardening only
NO_CHANGELOG=hardening, not a user-visible bug

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