Since we've changed CmdIsMaster to CmdHello in both replication_info and cluster_is_master_cmd (the latter file name is going to change), we should consider renaming classes like IsMasterOutcome to HelloOutcome.

This will help with maintaining consistency across various server classes that use metadata from the hello command. We will be doing something similar with IsMasterResponse (HelloResponse) and IsMasterMetrics (HelloMetrics).

We can also change IsMasterRTT to HelloRTT or even handshakeRTT.