improve low-fee/manually added tickets handling

[jolan]

Currently, it isn't clear in an exportable way which tickets have been added by the admin and which have not.

Need to do something like:

[ ] - add API/admin UI to dcrstakepool that allows adding/removing low-fee tickets to the DB
[ ] - prune table on dcrstakepool startup
[ ] - have stakepoold retrieve the list from the DB and cache it
[ ] - modify fee checks to consult the list