Refactor coio_task

* Add coio_task_create() and coio_task_destroy()
* Rename coio_task() to coio_task_post()
* Fix the core invariant of coio_task_post():
  - On timeout or when fiber was cancelled set diag, return -1 and
    guarantee that on_timeout will be called somewhen.
  - Otherwise don't touch diag, don't call on_timeout callback,
    return 0. Please check task->base.result and task->diag
    to get the original return code and diag from the callback.
* Change the return value of coio_task_post() to "int".
* Add diag to coio_getaddrinfo() and fix a possible bug in replication;
  ignore uncoventional getaddrinfo(3) error codes for now.
* Fix buggy box.snapshot() tests.

Needed for #1954