Sync up the total active urls value with state in UrlDbFunction

[vmagotra]

The total active urls is being maintained as a regular variable in UrlDbFunction.
Sync up the total active urls value with state whenever we restore from state.

[kkrugler]

Since state is keyed on PLD, to keep track of the actual total URLs (across all PLDs) we'd need to do something like leverage the _domainScores (unsaved) variable. When onTimer() is called, if an entry for the key (PLD) doesn't exist, then we increment our (also unsaved) _totalActiveUrls counter. Since timers are saved as state, they will eventually all fire, and then our counter will be accurate.