coio: fix getaddrinfo assertion on 0 timeout

Background. Coio provides a way to schedule arbitrary tasks
execution in worker threads. A task consists of a function to
execute, and a custom destructor.

To push a task the function coio_task_post(task, timeout) was
used. When the function returns 0, a caller can obtain a result
and should free the task manually. But the trick is that if
timeout was 0, the task was posted in a detached state. A
detached task frees its memory automatically despite
coio_task_post() result, and does not even yield. Such a task
object can't be accessed and so much the more freed manually.

coio_getaddrinfo() used coio_task_post() and freed the task when
the latter function returned 0. It led to double free when
timeout was set 0. The bug was introduced here
800cec73 in an attempt to do not
yield in say_logrotate, because it is not fiber-safe.

Now there are two functions: coio_task_execute(task, timeout),
which never detaches a task completed successfully, and
coio_task_post(task), which posts a task in a detached state.

Closes #4209