map/set/zset/list data_cf distributed compaction

[rockeet]

data_cf distributed compaction needs DB::Get from default cf in CompactionFilter, we bulk load all hash keys from default cf and save to shared filesystem for reading in dcompact workers.

When compaction input of dafa_cf is small but hash keys in default cf is large, this is a very big waste, --- when compact upper levels of data_cf, this may be likely happens.

So we should check data size in default cf before starting remote compaction, if it reaches a threshold of a percent of compaction input of dafa_cf, it should fallback to local compact.

1. Use Compaction::column_family_data() to get default cf handle and DB ptr, this needs a global std::map
2. Use DB::GetApproximateSizes() to get size in default cf.

Thus a customized CompactionExecutorFactory should be defined -- it should references DcompactEtcd factory and forward methods, the key point is to override ShouldRunLocal(compaction).