Fix race condition: do not mutate data without a mutex.

This fixes a data race discovered in Fingerprintd

WARNING: DATA RACE
Read by goroutine 21:
  bufio.(*Writer).flush()
      /Users/vaskozdravevski/code/go/src/pkg/bufio/bufio.go:527 +0xb6
  bufio.(*Writer).Flush()
      /Users/vaskozdravevski/code/go/src/pkg/bufio/bufio.go:519 +0x46
  github.com/sendgrid/go-statsdclient.(*client).Flush()
      /Users/vaskozdravevski/gocode/src/github.com/sendgrid/fingerprintd/Godeps/_workspace/src/github.com/sendgrid/go-statsdclient/statsd.go:151 +0x5d

Previous write by goroutine 24:
  bufio.(*Writer).Write()
      /Users/vaskozdravevski/code/go/src/pkg/bufio/bufio.go:575 +0x4cd
  fmt.Fprintf()
      /Users/vaskozdravevski/code/go/src/pkg/fmt/print.go:189 +0xd0
  github.com/sendgrid/go-statsdclient.(*client).send()
      /Users/vaskozdravevski/gocode/src/github.com/sendgrid/fingerprintd/Godeps/_workspace/src/github.com/sendgrid/go-statsdclient/statsd.go:189 +0x34a
  github.com/sendgrid/go-statsdclient.(*client).Duration()
      /Users/vaskozdravevski/gocode/src/github.com/sendgrid/fingerprintd/Godeps/_workspace/src/github.com/sendgrid/go-statsdclient/statsd.go:113 +0x191
@dtjm I didn't change this because it's existing behavior. Even before this PR, that is what would happen. Do you want me to fix this as part of this pull request, or should it be part of a bigger V3 fix? Tried to keep my PR to the minimum required to fix the race condition.