max users needs to be a config setting or project wide const

[chappjc]

We need to get around to making this a config setting at some point. Note that inaddition to the 10000 magic number appearing various config files for address derivation, it's also set as a package-level var in the controller package:

dcrstakepool/controllers/main.go

Line 46 in f6b21ee

MaxUsers = 10000

Originally posted by @chappjc in #308

[JoeGruffins]

I'll take this If that's ok.