Task management framework tries to stop a task twice (on master)

[mangalaman93]

alpha2    | panic: close of closed channel
alpha2    |
alpha2    | goroutine 332 [running]:
alpha2    | github.com/dgraph-io/badger/v2/y.(*Closer).Signal(...)
alpha2    | 	/home/paras/go/pkg/mod/github.com/dgraph-io/badger/v2@v2.0.1-rc1.0.20200316175624-91c31ebe8c22/y/y.go:205
alpha2    | github.com/dgraph-io/badger/v2/y.(*Closer).SignalAndWait(...)
alpha2    | 	/home/paras/go/pkg/mod/github.com/dgraph-io/badger/v2@v2.0.1-rc1.0.20200316175624-91c31ebe8c22/y/y.go:232
alpha2    | github.com/dgraph-io/dgraph/worker.(*node).startTask(0xc0002a61c0, 0x3, 0x0, 0x0, 0x0)
alpha2    | 	/home/paras/go/src/github.com/dgraph-io/dgraph/worker/draft.go:125 +0x1d2
alpha2    | github.com/dgraph-io/dgraph/worker.runSchemaMutation(0x1c0ee60, 0xc00003a0b8, 0xc0003222a0, 0x1, 0x1, 0x4, 0x0, 0x0)
alpha2    | 	/home/paras/go/src/github.com/dgraph-io/dgraph/worker/mutation.go:160 +0xd4
alpha2    | github.com/dgraph-io/dgraph/worker.(*node).applyMutations(0xc0002a61c0, 0x1c0ee60, 0xc00003a0b8, 0xc023092360, 0x0, 0x0)
alpha2    | 	/home/paras/go/src/github.com/dgraph-io/dgraph/worker/draft.go:315 +0xe5c
alpha2    | github.com/dgraph-io/dgraph/worker.(*node).applyCommitted(0xc0002a61c0, 0xc023092360, 0x289ab60, 0x17)
alpha2    | 	/home/paras/go/src/github.com/dgraph-io/dgraph/worker/draft.go:458 +0xe6b
alpha2    | github.com/dgraph-io/dgraph/worker.(*node).processApplyCh.func1(0xc0003b0a00, 0xe, 0x10)
alpha2    | 	/home/paras/go/src/github.com/dgraph-io/dgraph/worker/draft.go:587 +0x195
alpha2    | github.com/dgraph-io/dgraph/worker.(*node).processApplyCh(0xc0002a61c0)
alpha2    | 	/home/paras/go/src/github.com/dgraph-io/dgraph/worker/draft.go:628 +0x267
alpha2    | created by github.com/dgraph-io/dgraph/worker.(*node).InitAndStartNode
alpha2    | 	/home/paras/go/src/github.com/dgraph-io/dgraph/worker/draft.go:1547 +0x4bd