improve InternalStats output

Summary: as title

Test Plan:
sampe output:
Level Files Size(MB) Score Read(GB)  Rn(GB) Rnp1(GB) Write(BG) Wnew(GB) RW-Amp W-Amp Rd(MB/s) Wr(MB/s)  Rn(cnt) Rnp1(cnt) Wnp1(cnt) Wnew(cnt)  Comp(sec) Comp(cnt) Avg(sec) Stall(sec) Stall(cnt) Avg(ms)
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  L0    15     1024   1.0      0.0     0.0      0.0       8.2      8.2    0.0   0.0      0.0    111.4        0         0         1         1         75       123    0.612     295.94    1939238    0.15
  L1    23     2118   2.1     20.9     8.3     12.7      20.0      7.3    5.0   2.4     73.2     69.9      124       141       208        67        293         8   36.582      17.05      16100    1.06
  L2   162    15333   1.5     47.0     7.1     40.0      42.6      2.6   12.7   6.0     67.9     61.5       62       457       482        25        709        55   12.898       0.00          0    0.00
  L3   985   108065   1.1     37.8     4.0     33.9      36.9      3.0   18.8   9.3     60.1     58.5       41       338       363        25        645        31   20.812       0.00          0    0.00
  L4  2788   356033   0.3      0.0     0.0      0.0       0.0      0.0    0.0   0.0      0.0      0.0        0         0         0         0          0         0    0.000       0.00          0    0.00
 Sum  3973   482572   0.0    105.8    19.3     86.5     107.7     21.2   11.1   5.6     62.9     64.0      227       936      1054       118       1723       217    7.938     312.99    1955338    0.16

Reviewers: sdong, yhchiang, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19707