Provide a IDL definition for stream metadata and segment metadata

Currently there is not stream metadata structure and log segment metadata is a customized string structure. it would be good to use any IDL like thrift, protobuf to store the metadata like what bookkeeper is doing.

The reason to choose protobuf is

• because bookkeeper is using protobuf, it is okay to use same stuff.
• the thrift version that DL is using is a bit too old and hard to bump because it seems the version is customized for finagle. using protobuf is easier to support other language clients.