feat: poll barrier complete rpc in barrier manager worker loop

[wenym1]

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What's changed and what's your intention?

Previously in global barrier manager, the finish of barrier_complete rpc is notified by a mpsc channel, which may receive stale rpc result after recovery.

In this PR, we change to use a oneshot channel to notify the barrier completion of each epoch. The oneshot receiver will be pushed to a FuturesUnordered and gets polled in the global barrier manager worker loop. In this way, during failure recovery, we can avoid receiving stale completion result.

Checklist

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[yezizp2012]

LGTM~

[BugenZhao]

LGTM!

[wenym1]

Some methods related to barrier rpc is moved to rpc.rs without a line of change to make the code more organized.