Update converter API, user streams rather than filepaths

[afourney]

A very-large PR that updates DocumentConverters in a couple of major ways:

• split responsibilities of deciding if a file can be converted DocumentConverter.accepts() vs performing the conversion itself DocumentConverter.convert()
• file_stream, and stream_info inputs everywhere (rather than local_paths). No temporary files are used anymore.
• Mimetypes (in addition to file extensions) are considered when electing to convert a file 

Adapted from #1045 , Thanks @KennyZhang1

[gagb]

Fantastic PR. Thank you!