Ported seenUris MVar and activeWorkers MVar to TVar

TVar has a nicer API (i.e. a 'modifyTVar' function) and I believe it
will allow fixing a race condition I found.

Started using *Var suffix as well, now that MV doesn't fit anymore.
Fixed race condition in workerThread

The uriQueue and the activeWorkers variable should be updated.
Renamed activeWorkersTV to activeWorkersVar

For consistency.
Removed redundand 'do'