Make view and view_method nullable and shorter

[tomage]

I (finally!) just updated from a pre-1.0 version up to 1.4.0, and found that (1) it took over an hour to execute the migration and (2) it ate up almost 9GB of disk space. I do have about 12 million rows after all.

I figured, why not allow these values to be None/NULL, as it seems the right value to use for old entries, but it also kinda works for when the code can't figure out either the view name or the view method.

I.e. the answer to the question: "What view is this?" should be None, rather than the empty-string when the view can't be discerned, no?

[avelis]

@triat Would you have any objections to this? I don't but just wanted to run it by someone else just in case.

[triat]

@avelis So no I don't mind, my concern is more about the size of your DB growing which will, in any case, result in an impossible table to use because of the size. Therefore I would suggest using an ES maybe but that's not related to the issue.

DR; TL: it's ok for me

[tomage]

@triat Yeah - that's a definite concern coming up soon-ish (may start with pruning old entries).

Thanks for the approval guys, and thanks for the work you do on this library!