Replace all io_context* occurrences with io_context_t

Even in release mode die immediately when some io_* functions return
EINVAL. This always means some programming bug and it's better to fail fast.

LinuxAIOHandler::resubmit(): fix condition. Stop ignoring -1 return code which
corresponds to EPERM and io_submit() really can return this one.

Use io_destroy() to stop leaking io_context_t.

Make m_aio_ctx std::vector instead of C array. I think that internal check
for index overflow might be useful.

Add debug assertions for EFAULT because for me receiving it
looks like a programming bug.