Shutdown executor goroutines

[animesh2049]

We were not waiting for the executor goroutines to shutdown. Also we were trying to close the goroutines as soon as ctrl+c is hit.
This change would ensure that all the pending mutations are applied before goroutines are shutdown.
And we are also waiting for all such executor goroutines to finish before dgraph shuts down.

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This change is 

[manishrjain]

Can you mention the JIRA ticket this corresponds to?

Reviewed 1 of 2 files at r1, 1 of 1 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved

[jarifibrahim]

I see the following error on this branch line https://github.com/dgraph-io/dgraph/pull/5130/files#diff-e7949a0879bc67d5514e8a008bab05bfR909

0408 17:13:56.450455    9239 draft.go:911] =============ex nil %!(EXTRA bool=true)
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x14464d1]

goroutine 290 [running]:
github.com/dgraph-io/dgraph/worker.(*node).checkpointAndClose(0xc0002a0540, 0xc000424660, 0xc0002927e0)
	/home/ibrahim/Projects/go/src/github.com/dgraph-io/dgraph/worker/draft.go:912 +0x741
created by github.com/dgraph-io/dgraph/worker.(*node).Run
	/home/ibrahim/Projects/go/src/github.com/dgraph-io/dgraph/worker/draft.go:949 +0x1d0
[Sentry] 2020/04/08 17:13:56 Sending fatal event [17490d180a294aed80bd0bdbc820bc8b] to sentry.io project: 1805390
[Sentry] 2020/04/08 17:13:57 Buffer flushed successfully.

[animesh2049]

Thanks for pointing @jarifibrahim . My recent commit should have fixed the issue.