CLOUDSTACK-799: Redundant router: Speed up RvR status update

The basic idea behind this is, deploy a fix sized threadpool for updating RvR
status, then using producer/consumer model. There is a global configuration
router.check.poolsize(10 by default) to control the pool size.

Using pool size 100 for 1000 RvR is tested with simulator and works well.

Also we can adjust the global configuration option router.check.interval to e.g.
60s from default 30s to mitigate the issue.