add: check for available threads prior to submitting requests
add: check for available threads prior to submitting requests

* add `has_queued_threads` method that checks whether all threads are currently occupied
* `submit_documents`: check for available thread prior to submitting the request and wait for a specific time period if no threads are available.

weblyzard/weblyzard_api,weblyzard/weblyzard_api,weblyzard/weblyzard_api