Provide an option for configure multiple workers to apply DNS records changes

[sichenzhao]

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What would you like to be added:

Add a configurable worker counts, and use the configured number of workers to update diff records when things are changed.

This issue can be generalized for other resources as well, but I will volunteer to implement the resources we used first - istio gateway and istio virtual service

Proposed changes:

1. update here to multiple workers
2. update here to multiple workers as well

Why is this needed:

When there are more resources in k8s, currently the single threaded external-dns takes longer time to sync endpoints needed, similar to the problem reported here.

After a bit of debugging, we(me and @incfly) found that for loop like here takes pretty long time to finish.