cpu usage too high when write-busy

现象：某个写busy的同时，写pending达10w+；怀疑是remote_tablet的写线程接收请求过多，且发不到tablet write的写buffer，导致一直忙于应答写busy，进而cpu占用过高。

Start by tracing the remote_tablet write thread and the tablet write-buffer path described in the report. Reproduce the write-busy condition with more than 100,000 pending writes, then measure where CPU time is spent while requests receive write-busy responses. Done means the excessive CPU usage is explained and the write-busy handling no longer causes the reported load.

Written by the indexing model from the issue text.