Split reminds in time windows

[khaelys]

Looking at this we can do only 2000 transactions every 10 seconds. If the bot goes down and the cache is wiped up on Remind operation we can have serious problems.

Let's say that we want to notify all users in 5 minutes, we can guarantee that only for 60k users.

Originally posted by @khaelys in #34 (comment)

[simone3991]

I think this is a good thing to know but I suggest we take a look only when approaching that kind of user base, if ever.

[swissbyte]

good to have this in mind. I agree, if there are 60k users, there will definitely be other problems upcoming as well