improve performance for sequence read and warmup on big file

[zhoucheng361]

What would you like to be added:
bandwidth of object storage is 400M, the bandwidth of read is only 140M.
we have set buffer-size to 4G, but the process only use 40M buffer when reading.

root@bench-01:~/juicefs# /root/juicefs/juicefs mount tikv://172.27.0.1:2379/test1 /jfs -d --buffer-size 4096
root@bench-01:~/juicefs# md5sum /jfs/bigfile
root@bench-01:~/juicefs# ./juicefs stats /jfs

---------usage--------- ----------fuse--------- -------------meta------------ -blockcache ---------------------object--------------------
 cpu   mem   buf  cache| ops   lat   read write| ops   lat   txn   lat  retry| read write| get  get_c  lat   put  put_c  lat  del_c  lat
35.5%  271M   36M    0 |1142  2.63   141M    0 |   5  0.71     0     0     0 |   0   144M| 144M   36   158     0     0     0     0     0
29.2%  271M   36M    0 | 990  3.14   121M    0 |   6  0.80     0     0     0 |   0   112M| 112M   28   166     0     0     0     0     0
40.8%    0    32M 8192K|1294  1.96   160M    0 |   5  0.77     0     0     0 |   0   156M| 164M   41   145     0     0     0     0     0
25.4%  271M   36M    0 | 766  4.85    94M    0 |   3  0.64     0     0     0 |   0   112M| 104M   26   171     0     0     0     0     0
37.0%  271M   36M    0 |1284  2.23   158M    0 |   5  0.75     0     0     0 |   0   152M| 152M   38   152     0     0     0     0     0
33.3%  265M   32M    0 | 922  3.47   113M    0 |   4  0.77     2  2.08     0 |   0   128M| 128M   32   187     0     0     0     0     0
29.5%  267M   40M    0 |1116  2.68   137M    0 |   6  0.68     0     0     0 |   0   112M| 112M   28   184     0     0     0     0     0
39.7%  267M   36M 4096K|1206  2.47   150M    0 |   4  0.76     0     0     0 |   0   160M| 164M   41   155     0     0     0     0     0


root@bench-01:~/juicefs# ./juicefs warmup /tmp/jfs/bigfile
warmup cache file: 0                             0.0/s                          ⠹
warmup cache size: 192.0 MiB (201326592 Bytes)   21.9 MiB/s                     ⠹

root@bench-01:~# dstat
You did not select any stats, using -cdngy by default.
--total-cpu-usage-- -dsk/total- -net/total- ---paging-- ---system--
usr sys idl wai stl| read  writ| recv  send|  in   out | int   csw
  2   1  97   0   0| 325k 2303k|   0     0 |   0     0 |2457    36k
  2   1  96   2   0|4096B   82M|  25M  150k|   0     0 |  14k   36k
  2   1  97   0   0|   0    16k|  17M  120k|   0     0 |  12k   34k
  1   1  97   0   0|   0   264k|  13M   81k|   0     0 |  11k   32k
  2   1  97   0   0|   0  8192B|  17M  100k|   0     0 |  11k   33k
   ⠋

Why is this needed: