Control chunkserver memory

- Use flag to config chunkserver's max sliding window size. If cs has reached it's limit, we still allocate slid in opened window, but do not open new windows.
- Reduce sliding window size to 10
- SDK may store more data than the size of sliding window, but only data slides within the window uper-bound can be sent.
- When computing the load score for cs, ns should take number of sliding windows into consider.
- 通过flag配置chunkserver允许的最大滑动窗口数（即最大打开文件数），超过上限后，旧的窗口中可以新加入数据分片，但是不允许打开新的窗口
- 减小单个滑动窗口的大小到10
- sdk端可以囤积比滑动窗口更大的数据量，但是发送时只允许发送窗口允许的数据分片
- nameserver端在对chunkserver进行打分时，尚未加入chunkserver上现有滑动窗口个数的因素

Start by tracing sliding-window handling in the chunkserver, SDK send bounds, and nameserver chunkserver load scoring; no specific files or tests are named. Done means the maximum window count is flag-configurable, the window size is 10, SDK data beyond the active window is retained but not sent, and nameserver scoring includes the current window count.

Written by the indexing model from the issue text.