serf: expose a protocol version from Serf
serf: keep track of the protocol verisons of each member
command/agent: can specify the protocol config
command/agent: fix docs
command: add protocol to version command
command: add "-detailed" to `serf members` to see the protocol versions
website: document the protocol versions
website: document -detailed flag
serf: Add ProtocolVersion method
serf: default protocol is max
serf: static map for protocl version mapping