Feature Request: Add support of mongodb store

[amismailz]

Hi,

Firstly, I'd like to thank you so much for your extremely helpful module.

In my case, the site I am working on has large number of pages, so, in-memory solutions would not be suitable for me.

If this supports mongodb cache store, this will be a very big adding I believe.

[asiainfoliwei]

mongodb is so slow，recommend using an in memory database