PeriodicDispatch.SetEnabled sets updateCh in one goroutine, and
PeriodicDispatch.run accesses updateCh in another.
The race can be prevented by having SetEnabled pass updateCh to run.
Race detector output from `go test -race -run TestServer_RPC` in nomad/
```
==================
WARNING: DATA RACE
Write at 0x00c0001d3f48 by goroutine 75:
github.com/hashicorp/nomad/nomad.(*PeriodicDispatch).SetEnabled()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/periodic.go:468
+0x256
github.com/hashicorp/nomad/nomad.(*Server).revokeLeadership()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/leader.go:724
+0x267
github.com/hashicorp/nomad/nomad.(*Server).leaderLoop.func1()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/leader.go:131
+0x3c
github.com/hashicorp/nomad/nomad.(*Server).leaderLoop()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/leader.go:163
+0x4dd
github.com/hashicorp/nomad/nomad.(*Server).monitorLeadership.func1()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/leader.go:72
+0x6c
Previous read at 0x00c0001d3f48 by goroutine 515:
github.com/hashicorp/nomad/nomad.(*PeriodicDispatch).run()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/periodic.go:338
+0x177
Goroutine 75 (running) created at:
github.com/hashicorp/nomad/nomad.(*Server).monitorLeadership()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/leader.go:70
+0x269
Goroutine 515 (running) created at:
github.com/hashicorp/nomad/nomad.(*PeriodicDispatch).SetEnabled()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/periodic.go:176
+0x1bc
github.com/hashicorp/nomad/nomad.(*Server).establishLeadership()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/leader.go:231
+0x582
github.com/hashicorp/nomad/nomad.(*Server).leaderLoop()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/leader.go:117
+0x82e
github.com/hashicorp/nomad/nomad.(*Server).monitorLeadership.func1()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/leader.go:72
+0x6c
==================
```
