Added MMapIndexedCache

[OhadRubin]

#4591
This is a really rough draft...

[OhadRubin]

Hey, @epwalsh, i'm in the process of adapting the original fairseq code to our needs.
Right now the structure of the classes if kinda weird with _Writer and Index (from fairseq).
Can you review what I have here so far?

[epwalsh]

Hey @OhadRubin, is there any reason why the Index class needs to be defined within the MMapCacheReader class?

[OhadRubin]

No, that's one of the things from fairseq I kinda want to change...

[OhadRubin]

Btw, during instance creation, is it possible to require that all fields exists, this will really help deserialization.
Where in "test set" situation, where there isn't any supervision, the user passes a empty field, or something along that.
This will allow us to use things that expect a certain structure (schema) like pyarrow here