maxusers: Update TODOs

[JoeGruffins]

-make max users a configurable variable
-default 10000
-stakepoold start with default 10000
-when starting dcrstakepool grpc commands are used to set the max on all servers and re-derive pool fees if there was a change

ahh, meant to do the draft thing, adding WIP to title

This pr took a different direction mid-stream. I now think that maxusers is an artificial ceiling and it should be removed. If my understanding is incorrect, please comment.

[JoeGruffins]

I think this is finished. It will change a bit after #463 if that goes through first.

[JoeGruffins]

Will rebase after release.

[JoeGruffins]

@jholdstock Let me know your thoughts:
Q. What is max users there for?
My A. To decide the number of fee addresses that should be derived.
Q. Should max users be settable?
A. Yes. As dcrstakepool is now, a pool that is approaching max users must be able to increase this to allow more users.
Q. Should it be set in one place?
A. No preference from me. But I believe you think this could lead to bugs later on. In which case the appropriate path to take is add maxusers to the stakepoold conf and check that they are all the same on startup. If not let the operator change his/her conf.
Q. What's the future of max users?
A. Lets remove it. I believe there was talk yesterday on setting all of the fee addresses inside dcrwallet. It shouldn't be done there. Having a limit on users because of the feeaddresses (if that is the reason for this config setting) is silly.

Edit: I just remembered this is also set in the dcrwallet conf.

[JoeGruffins]

This pr won't work. stakepoolcoldextkey must also be set on wallet start-up and there's no way to query/set the value afterwards. As I think this entire concept will be changed in the end it's probably not worth making this value settable through rpc commands. As for this pr and the issue, either we should make it settable also in stakepoold conf or make it a constant for now. currently max users can't exceed 10000, even if set above, so making it a constant should not have an affect on any current pools.

[JoeGruffins]

If there's no good way to check that dcrwallet.conf value, I think making this settable is a bad idea. I'm going to make maxusers a constant unless there is an objection.

[JoeGruffins]

Ok. I think #310 should be closed in order to progress in the direction of #504.

[jholdstock]

Sorry this one got ignored for so long. I've read your braindumps above and I agree with the gist of it all, so lets just merge this one and look back at this once we have privacy implemented.