[bug fix] Need to wait for previous recover task finished.

[liuyu85cn]

What type of PR is this?

• bug
• feature
• enhancement

What problem(s) does this PR solve?

Issue(s) number:

n/a

Description:

In TOSS, each time a raft part get leader, it will do a scan to see if there is any primes.
but if the leader changed really quick. There may be some previous term scan not finished. yet.
These to recover task will write the same key and finished in any order.

In this PR, the latest recover task will wait for previous task finish, then start.

How do you solve it?

Special notes for your reviewer, ex. impact of this fix, design document, etc:

Checklist:

Tests:

• Unit test(positive and negative cases)
• Function test
• Performance test
• N/A

Affects:

• Documentation affected (Please add the label if documentation needs to be modified.)
• Incompatibility (If it breaks the compatibility, please describe it and add the label.）
• If it's needed to cherry-pick (If cherry-pick to some branches is required, please label the destination version(s).)
• Performance impacted: Consumes more CPU/Memory

Release notes:

n/a

[panda-sheep]

Good job！

[wenhaocs]

excellent

[wenhaocs]

minor: "pro" usually means good vs. con. It's better to use "p", "promise" or "xxPromise" to indicate promise.