Move document_name attribute to meta

[tanaysoni]

This PR makes all three document stores(Elasticsearch, SQL, and InMemory) compliant in how they ingest documents using the write_documents() method.

The input dicts for write_documents() have a mandatory text field. All other metadata used for filtering documents(eg, name, author, category etc) can be passed under a meta dict. So, a sample dict would look like this:

{
    "text": "....body-of-the-document...",  # mandatory
    "meta": {  # meta is optional
         "year": "2019",  # all values are strings
         "name": "xyz",
         "author": "abc"
    }
}

For backward compatibility, the meta fields can also be passed as top-level keys in the document dict. For example,

{
    "text": "....body-of-the-document...",  # mandatory
    "name": "abc"  # optional; this will get interepreted as a `meta` field
}

[tholor]

If I got this right we are making the document stores more similar for the input like {"text": "<the-actual-text>", "meta":[{"name": "some name"}]}.
What about input like{"text": "<the-actual-text>", "some_meta_field": "value"...}? I believe this is still treated differently. For elasticsearch we will "collect" these extra fields in "meta", while we don't do this for InMemory and SQL. Right?