remote_bootstrap_client: mild API changes

1. I removed the uuid argument from the constructor; it's always the uuid of
   the FsManager.
2. I made the TabletStatusListener argument to FetchAll() optional. I think
   that was the original intent (because the TabletMetadata OUT parameter in
   Start() is optional), but a CHECK_NOTNULL() got added at some point.
3. I removed the peer uuid argument from Start(). It was only used for
   logging, so I added an equivalent field to the response protocol.

Change-Id: Id7f482d1193a5a46fa51488eab4712c0c4bfceb1
Reviewed-on: http://gerrit.cloudera.org:8080/3811
Tested-by: Kudu Jenkins
Reviewed-by: Todd Lipcon <todd@apache.org>
7 files changed