cherry-pick following patches
```
ff30897d 2019-09-23 zbk602423539@gmail.. reset gc mark after sampling (#88)
cb67cd56 2019-09-22 zbk602423539@gmail.. fix wrong property after restart (#82)
c6e408c2 2019-09-20 yiwu@pingcap.com     Fix blob file format description (#83)
fc4106d4 2019-09-18 yiwu@pingcap.com     Fix new CF missing BlobFileSizeCollector (#78)
10710bb1 2019-09-18 zbk602423539@gmail.. Add more metrics (#79)
8768067e 2019-09-17 yiwu@pingcap.com     Prevent CF being dropped while GC is running (#72)
07aa0655 2019-09-16 zbk602423539@gmail.. Rename version_set to blob_file_set (#69)
f963b880 2019-09-16 zbk602423539@gmail.. support histogram for titan stats (#74)
144e20f0 2019-09-12 wujy.cs@gmail.com    gc breakdown metrics (#73)
715dbd69 2019-09-11 zbk602423539@gmail.. Update BlobFileMeta format (#68)
468ddc97 2019-09-03 yiwu@pingcap.com     Add internal operation stats and dump to info log periodically (#62)
```