Support graceful shutdown for scheduler.

[zhzy0077]

Which issue does this PR close?

Closes #565.

Rationale for this change

What changes are included in this PR?

Setting the terminating flag when receiving terminate signals and shutdown the server gracefully.

Are there any user-facing changes?

No.

N.B. I'm really a newbie to Rust. Appreciate in advance for your reviewing the PR and raising comments.

[mingmwang]

@liukun4515