Optimize index loading time

[thammegowda]

Now that index is getting big, we need to think about optimizing the loading of the index.

We can pickle dump index object for the first time and load from it for subsequent runs.
I am worried about a pickled object going out of sync with the code (hence I didn't do it at the first place)

Idea1: use version number -- if version number doesn't match, then rebuild the index.

But what do we do about the development version where the index is currently being edited?

idea2: add a CLI flag to force update index