Summary:
Rocksdb backup engine maintains metadata about backups in separate files. But,
there was no way to add extra application specific data to it. Adding support
for that.
In some use cases, applications decide to restore a backup based on some
metadata. This will help those cases to cheaply decide whether to restore or
not.
Test Plan:
Added a unit test. Existing ones are passing
Sample meta file for BinaryMetadata test-
```
1459454043
0
metadata 6162630A64656600676869
2
private/1/MANIFEST-000001 crc32 1184723444
private/1/CURRENT crc32 3505765120
```
Reviewers: sdong, ldemailly, andrewkr
Reviewed By: andrewkr
Subscribers: andrewkr, dhruba, ldemailly
Differential Revision: https://reviews.facebook.net/D56007
b55e2165be