Allow to fetch only x months of history since last fetch

I have a lot (3500+) transactions retrieved at each fetch to my bank's website, since its creation date. This take quite a lot of time (5+ minutes) to retrieve all of this whereas there is no chance an operation is created between two others in the past.

If at each fetch we would request only the operations that happened from 1 month before the last fetch (a 1 month gap seems OK to ensure we don't miss any transaction which is late to appear), that would reduce the number of requests we'd have to make. On first fetch there is no last fetch date so we simply fetch everything.

I think we could use a setting in the config file to customize the gap, 0 would mean to disable the feature, 1 for one month, etc.

For example boobank already does it: https://git.weboob.org/weboob/weboob/blob/master/weboob/applications/boobank/boobank.py#L547

We'd have to use an iterator here instead of a list() https://framagit.org/kresusapp/kresus/blob/master/server/weboob/main.py#L525.

@all What do you think?