[BUG] Fix Colocate table balance bug #4936
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kangkaisen
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This was referenced Nov 24, 2020
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Fix bug #4935
当前策略：
每个group中维护一个bucketId所在的be列表：backendSeq
线程每隔20s：
检测backendSeq中是否有be不可用，如果有，则选择可用的be将其在backendSeq中替换
检测group中的tablet是否与backendSeq相匹配，如果不匹配，将group设置为unstable，并且执行迁移任务
对处于stable状态的group进行均衡：根据backendSeq计算所有be中bucketId的数目，从bucketId占有高的be迁移到bucketId占有低的be。此处只更新backendSeq，实际执行迁移任务在第2步。
存在的问题：
如果在相同的时间down掉比较多的be，在第1步中，会将这些be从backendSeq中移除，并且第2步检测到backendSeq不匹配，将group标记为unstable，但是如果现有的be磁盘不能容纳down掉的be上的所有tablet，此时group会一直处于unstable状态，即使再加入新的be，也不能触发第3步，因为第3步只会在group是stable状态下才能执行。
策略更改：
将现有策略的1和3融合成一个过程：
首先检测backendSeq中是否存在不可用的be，均衡时，优先迁移不可用be的bucketId到buckedId占有低的be，其次再从bucketId占有高的be迁移到bucketId占有低的be。
同现有策略