Add MaxLength

[mmyoungman]

Hi,

Thanks for writing sqlitestore.

I had been hitting this issue while using openid-connect`as a provider with goth: markbates/goth#133

The solution is to increase the size of SecureCookies, which default to 4096. To do this with sqlitestore, I've added a new MaxLength method. This is basically a copy from github.com/gorilla/sessions's FilesystemStore's MaxLength: https://github.com/gorilla/sessions/blob/466d29e7f343560836292b7c922e1385e908ef95/store.go#L173

Staticcheck was showing a couple of warnings, so I also fixed them.

Thanks,
Mark