iv_work: add support for submitting work_items from within worker threads

[bazsi]

This is a request-for-comments PR against iv_work to allow the submission of tasks from a worker thread and not just from the thread that owns the iv_work pool.

[MrAnno]

This is a really interesting addition that we could build upon in syslog-ng.

I wonder how this approach fits into the ivykis world.

The behavior of the completion callback might be surprising when submitting jobs from a worker, but it does what is currently documented.

[buytenh]

I've rebased this commit onto master and merged it as 7263f81301d2, where it is part of the v0.43 release. Thank you!