Move distributedlog build to depend on central maven repo

Reported https://github.com/twitter/distributedlog/issues/37.

Currently they are a few dependencies only exists at maven.twttr. There are two major pieces, one is the finagle-serverset and while the other one is thrift library. The finagle-serverset is majorly used for service-discovery by distributedlog-client and thrift is used by scrooge and the rpc wire protocol between client and write proxy.

There are two options:

• one is to look for a solution to move those dependencies to central maven repo.
• the other one is to see if we can make thrift and its service discovery pluginnable. so we can support both finagle-thrift and gRPC-protobuf.