Adding guidance of configuring compaction related parameters

[mitake]

I did some performance evaluation and found that too short compaction is harmful for throughput but too long compaction also degrades throughput. The result can be found in page 49 to 51 of https://www.slideshare.net/mitakeh/understanding-performance-aspects-of-etcd-and-raft

Based on profiling with pprof, I found that some functions related to memory allocation and garbage collection (e.g. runtime.mallocgc and runtime.scanobject) are consuming much more cycles when the compaction interval becomes larger. It can be considered that the mark phase of GC becomes heavy because of the increased number of live objects (in this case, raft.MemoryStorage.ents would be a dominating object).

Although the difference is not so significant, adding guideline for performance tuning would be helpful because the long interval is not good for memory consumption and CPU cycles.

/cc @xiang90 @heyitsanthony this is what I talked today

[gyuho]

@mitake Link is broken?

EDIT: seems like slideshare issue. Works with different links.

[mitake]

@gyuho really? I could see the link even without login to slideshare. Did you see 404?

[fanminshi]

@gyuho link works for me

[heyitsanthony]

Related: #7607

[mitake]

@fanminshi thanks for checking!
@gyuho if you still see the problem, please let me know.