Apply committed Raft entries in a separate thread

[ffuugoo]

This PR applies committed Raft entries in a separate thread (using RawNode::advance_append and RawNode::advance_apply_to). This should considerably speed up consensus thread...

Currently based on #3964, will be rebased on dev once it's merged.

All Submissions:

• Contributions should target the dev branch. Did you create your branch from dev?
• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

1. Does your submission pass tests?
2. Have you formatted your code locally using cargo +nightly fmt --all command prior to submission?
3. Have you checked your code using cargo clippy --all --all-features command?

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?

[ffuugoo]

When using no-ops (finish non-existing shard transfer) as the simplest test, consensus is doing 10k ops / second. 🤯🤯🤯