GitLab

- do not keep track of a single time variable for whole structure change
- save to masternode.dat and clear on checks:
  - who's asked for the masternode list and the last time
  - who we asked for the masternode list and the last time (will help to prevent dseg ban when restart wallet several times in short period of time)
  - which masternodes we've asked for
- do not use timeout for CheckAndRemove (played with that - doesn't make sense actually)