ServerStats should hold time.Durations for latencies, not int64

[langston-barrett]

Currently, ServerStats holds

type ServerStats struct {
    Sent        int64
    Received    int64
    NodeCount   int64
    MinLatency  int64
    AvgLatency  int64
    MaxLatency  int64
    Connections int64
    Outstanding int64
    Epoch       int32
    Counter     int32
    BuildTime   time.Time
    Mode        Mode
    Version     string
    Error       error
}

Whereas this should probably be

type ServerStats struct {
    Sent        int64
    Received    int64
    NodeCount   int64
    MinLatency  time.Duration
    AvgLatency  time.Duration
    MaxLatency  time.Duration
    Connections int64
    Outstanding int64
    Epoch       int32
    Counter     int32
    BuildTime   time.Time
    Mode        Mode
    Version     string
    Error       error
}