Set Default LastFollowerDate for Channels in FollowerService

[benrick]

Feel free to use this, toss it, or just reference what I'm talking about from it.

This is associated with #87

[RRickkert]

A bit a late response, but I just checked it again and understand what your approach is there! Smart thinking! That would still have to be added to the 'SetChannelsById' method though, and would allow us to remove the 'bool callEvents = true' param in the UpdateLatestFollowersAsync method. I recently also noticed that I didn't add a clear method to basically say InitializeLatestFollowersAsync(), which was supposed to call UpdateLatestFollowersAsync(false), and so "initialize the latest followers", removing the need for your code. But your way of working removes the need for that call! Nice thinking!