feat: add partition guardian to keep one primary and two secondaries

[levy5307]

I think some functions in simple_load_balancer have nothing to do with load balancer, such as: cure/collect_replica/construct_replica, etc. So I want to remove them eventually.

In this pull request, A class named with partition_guardian is added, which job is to keep one primary and two secondaries. And move simple_load_balancer::cure into it.

And I will move collect_replica and construct_replica to be a function outside class later.

related issue: #848

[Smityz]

LGTM

[hycdong]

I suggestion that you can add an issue in pegasus repo like issue 141 or issue 766 to track your refactor.

[levy5307]

I suggestion that you can add an issue in pegasus repo like issue 141 or issue 766 to track your refactor.

Done

[foreverneverer]

collect_replica actually like util method, and create new class for one method is not appropriate, maybe you can consider the refactor again

[levy5307]

collect_replica actually like util method, and create new class for one method is not appropriate, maybe you can consider the refactor again

good idea. I will move collect_replica/construct_replica to be a method outside class