block eta

[roadscape]

Currently there is a loop which checks dgp until expected block is detected. This makes a lot of calls to jussi (and returned data is imprecise because calls cached by 3s). Each call can be up to 100-200ms which is a significant waste. Since blocks are at regular 3s intervals, hive should just request blocks at their ETA.

Also, once we have a known amount of idle time between blocks, it can be used for maintenance.