This fixes a classic concurrency bug where the stop channel
was being listened in 2 places. In the Run method and DoJob
method.
The problem with that is that one of them will consume
the signal, and if the DoJob method does, then the parent
for-select loop never exits, because the return statement
only runs when the stop signal is consumed.
This would lead to server hanging on shutdown.
To fix this, we close the channel rather than sending a
message. This exits all listeners listening on the channel
and lets the job exit gracefully.
This is a sample stack dump of a stuck goroutine
```
goroutine 1 [chan receive]:
runtime.gopark(0x235be40?, 0xc000faa530?, 0xa8?, 0xf3?, 0x27a1170?)
/home/agniva/gosource/go/src/runtime/proc.go:366 +0xd6 fp=0xc002e77510 sp=0xc002e774f0 pc=0x43c856
runtime.chanrecv(0xc000faa7e0, 0x0, 0x1)
/home/agniva/gosource/go/src/runtime/chan.go:577 +0x56c fp=0xc002e775a0 sp=0xc002e77510 pc=0x40868c
runtime.chanrecv1(0x27cab4c?, 0xf?)
/home/agniva/gosource/go/src/runtime/chan.go:440 +0x18 fp=0xc002e775c8 sp=0xc002e775a0 pc=0x4080b8
github.com/mattermost/mattermost-server/v6/jobs/migrations.(*Worker).Stop(0xc001954280)
/home/agniva/mattermost/mattermost-server/jobs/migrations/worker.go:66 +0x171 fp=0xc002e77698 sp=0xc002e775c8 pc=0x187f8d1
github.com/mattermost/mattermost-server/v6/jobs.(*Workers).Stop(0xc0019df2c0)
/home/agniva/mattermost/mattermost-server/jobs/workers.go:84 +0x12c fp=0xc002e77748 sp=0xc002e77698 pc=0x17b04ec
github.com/mattermost/mattermost-server/v6/jobs.(*JobServer).StopWorkers(0xc00021f180?)
/home/agniva/mattermost/mattermost-server/jobs/server.go:104 +0x9c fp=0xc002e777a0 sp=0xc002e77748 pc=0x17af8bc
github.com/mattermost/mattermost-server/v6/app.(*Server).Shutdown(0xc00021f180)
/home/agniva/mattermost/mattermost-server/app/server.go:1019 +0x965 fp=0xc002e77c48 sp=0xc002e777a0 pc=0x1dd6865
github.com/mattermost/mattermost-server/v6/cmd/mattermost/commands.runServer.func3()
/home/agniva/mattermost/mattermost-server/cmd/mattermost/commands/server.go:79 +0x26 fp=0xc002e77c60 sp=0xc002e77c48 pc=0x214bbe6
github.com/mattermost/mattermost-server/v6/cmd/mattermost/commands.runServer(0xc000e1e000, 0xb?)
/home/agniva/mattermost/mattermost-server/cmd/mattermost/commands/server.go:122 +0x239 fp=0xc002e77d50 sp=0xc002e77c60 pc=0x214ba39
github.com/mattermost/mattermost-server/v6/cmd/mattermost/commands.serverCmdF(0x4207ac0?, {0x279d9d6?, 0x0?, 0x0?})
/home/agniva/mattermost/mattermost-server/cmd/mattermost/commands/server.go:58 +0x11d fp=0xc002e77dd0 sp=0xc002e77d50 pc=0x214b37d
github.com/spf13/cobra.(*Command).execute(0x4207ac0, {0xc00003c200, 0x0, 0x0})
/home/agniva/mattermost/mattermost-server/vendor/github.com/spf13/cobra/command.go:856 +0x67c fp=0xc002e77ea8 sp=0xc002e77dd0 pc=0x211a21c
github.com/spf13/cobra.(*Command).ExecuteC(0x4207ac0)
/home/agniva/mattermost/mattermost-server/vendor/github.com/spf13/cobra/command.go:974 +0x3b4 fp=0xc002e77f60 sp=0xc002e77ea8 pc=0x211a894
github.com/spf13/cobra.(*Command).Execute(...)
/home/agniva/mattermost/mattermost-server/vendor/github.com/spf13/cobra/command.go:902
github.com/mattermost/mattermost-server/v6/cmd/mattermost/commands.Run(...)
/home/agniva/mattermost/mattermost-server/cmd/mattermost/commands/root.go:14
main.main()
/home/agniva/mattermost/mattermost-server/cmd/mattermost/main.go:31 +0x86 fp=0xc002e77f80 sp=0xc002e77f60 pc=0x21f9ee6
runtime.main()
/home/agniva/gosource/go/src/runtime/proc.go:255 +0x227 fp=0xc002e77fe0 sp=0xc002e77f80 pc=0x43c487
runtime.goexit()
/home/agniva/gosource/go/src/runtime/asm_amd64.s:1571 +0x1 fp=0xc002e77fe8 sp=0xc002e77fe0 pc=0x46d361
```
This is not strictly related to the JIRA ticket but
a general refactor
https://mattermost.atlassian.net/browse/MM-40272
```release-note
NONE
```
29b78f84d7