Use standard format for backup files

[xumengpanda]

If backup files use a standard format, such as Protocol Buffers or FlatBuffers, digesting the backup will not need the verbose and error-prone deserialization process.

It will make life easier to inspect backup data using external tools, such as Spark.

Since we are going to have a new backup system, it worth investigating how hard it is to incorporate this into the new backup system and the new performant restore system.

[This is a offline discussion with @apkar.]