Speed up handler creation

[cw9]

When there are hundreds of hosts configured for a flush handler backend, it could take 8-10s to create all the connections to them, in some cases we have 6-7 handlers defined with that many hosts, it will take the process a very long time to start up.

We should try a few things to speed up the handler construction to shorten the process start up time:
1, parallelize the handler creation.
2, use ip instead of hostname in the backend configuration to avoid DNS lookup.