Update qdrant interface

[joein]

Hello

1. Passing embedding_function as a callable seems to be outdated and the common interface is to pass Embeddings instance

2. At the moment Qdrant.add_texts is designed to be used with embeddings.embed_query, which is 1) slow 2) causes ambiguity due to 1. It should be used with embeddings.embed_documents

This PR solves both problems and also provides some new tests

[joein]

Hi, @hwchase17 Great, thanks I’ve just fixed linter complaints, it should work now