dfdaemon slow download promblem in long run

[bigerous]

Bug report:

I did some press test on 990 machines. I found a strange thing:the performance is better if I restart dfdaemon before dfget download. There may be some bugs.
Here is the test report:

---

restart dfdaemon before dfget download:

文件大小	节点数量	成功率	时间	avg rate/s	max rate/s	min rate/s
10K	1	100.0%	1.757s	5.692KiB	5.692KiB	5.692KiB
10K	5	100.0%	2.678s	4.352KiB	5.824KiB	3.734KiB
10K	10	100.0%	2.585s	4.972KiB	6.698KiB	3.868KiB
10K	100	100.0%	2.979s	4.904KiB	7.348KiB	3.357KiB
10K	990	100.0%	4.201s	4.73KiB	6.192KiB	2.38KiB
200M	1	100.0%	4.142s	48.29MiB	48.29MiB	48.29MiB
200M	5	100.0%	3.644s	61.91MiB	65.38MiB	54.88MiB
200M	10	100.0%	5.267s	47.32MiB	62.31MiB	37.97MiB
200M	100	100.0%	6.304s	42.05MiB	67.2MiB	31.73MiB
200M	990	100.0%	9.246s	36.37MiB	78.09MiB	21.63MiB
1G	1	100.0%	11.669s	87.75MiB	87.75MiB	87.75MiB
1G	5	100.0%	17.852s	71.21MiB	93.27MiB	57.36MiB
1G	10	100.0%	24.36s	62.65MiB	95.86MiB	42.04MiB
1G	100	100.0%	43.314s	31.04MiB	96.79MiB	23.64MiB
1G	990	100.0%	28.492s	49.26MiB	84.89MiB	35.94MiB
10G	1	100.0%	1m42.622s	99.78MiB	99.78MiB	99.78MiB
10G	5	100.0%	4m26.479s	48.34MiB	86.56MiB	38.43MiB
10G	10	100.0%	5m3.177s	54.91MiB	89.56MiB	33.78MiB
10G	100	100.0%	6m57.945s	29.32MiB	63.79MiB	24.5MiB
10G	990	100.0%	6m28.227s	34.74MiB	81.03MiB	26.38MiB

---

no restart before dfget download:

文件大小	节点数	成功率	总耗时	avg rate/s	max rate/s	min rate/s
10K	1	1.0	2.254s	4.437KiB	4.437KiB	4.437KiB
10K	5	1.0	2.735s	4.2KiB	4.462KiB	3.656KiB
10K	10	1.0	2.956s	4.082KiB	4.941KiB	3.383KiB
10K	100	1.0	4.688s	3.942KiB	5.368KiB	2.133KiB
10K	990	1.0	5.014s	150.7KiB	1.953MiB	1.994KiB
200M	1	1.0	1.057s	189.2MiB	189.2MiB	189.2MiB
200M	5	1.0	1.425s	149.7MiB	155.8MiB	140.4MiB
200M	10	1.0	7.758s	40.74MiB	138.4MiB	25.78MiB
200M	100	1.0	7.714s	31.17MiB	188.1MiB	25.93MiB
200M	990	1.0	29.71s	10.26MiB	61.67MiB	6.732MiB
1G	1	1.0	9.983s	102.6MiB	102.6MiB	102.6MiB
1G	5	1.0	22.66s	61.53MiB	110.1MiB	45.19MiB
1G	10	1.0	37.031s	37.62MiB	100.6MiB	27.65MiB
1G	100	1.0	42.15s	29.48MiB	111.6MiB	24.29MiB
1G	990	1.0	2m11.768s	10.59MiB	84.47MiB	7.771MiB
10G	1	1.0	1m51.365s	91.95MiB	91.95MiB	91.95MiB
10G	5	1.0	4m25.247s	52.18MiB	90.03MiB	38.61MiB
10G	10	1.0	6m42.122s	35.63MiB	95.58MiB	25.46MiB
10G	100	1.0	7m40.467s	23.17MiB	92.78MiB	22.24MiB
10G	990	1.0	11m4.111s	19.52MiB	90.37MiB	15.42MiB

Expected behavior:

How to reproduce it:

Environment:

• Dragonfly version:
• OS:
• Kernel (e.g. uname -a):
• Others: