implement backup descriptor for incremental backup

[qiyanghe1998]

Follow Qiyang He's design for incremental backup, this diff adds the backup descriptor for each backup.

Generally, it will create a folder for each backup distinguished by the timestamp. Under each folder, there will be newly added SST files (including those from compaction), backup descriptor, CURRENT, MANIFEST and OPTION. The backup descriptor contains a map from SST filename to timestamp and the range of SST files. You can refer to https://docs.google.com/document/d/1aRO1PJ8qUN-yn6iUK7cisk7qSAsKFhnoyig0_T89jTo/edit#heading=h.oxp5c6sx5wxx for more details.

[qiyanghe1998]

Addressed all the comments. Could you have another look? @mradwan Thanks!

[qiyanghe1998]

Pushed a new version with backup starter