Weaviate: add missing async methods

[anakin87]

Some async methods are missing from the Weaviate Document Store:

• count_documents_async
• filter_documents_async
• write_documents_async
• delete_documents_async
• delete_all_documents_async

These methods should be developed, trying to reduce code duplication with their synchronous versions (extracting common private functions is allowed).

Also asynchronous tests should be adapted/added to actually test these async methods.