Data Race in skysocks

[ersonp]

Describe the bug
A clear and concise description of the problem and bug effects.
Data race occours when a visor built with -race flag is closed with Ctrl+C.

Environment information:

• OS: Linux
• Platform: Linux 5.8.0-59-generic x86_64

Steps to Reproduce
Steps to reproduce the behavior:

1. Build skywire with -race flag
2. Config must have skysocks app
3. Start the visor
4. Ctrl + C

Actual behavior

[2021-07-14T22:10:39+05:30] INFO [9/10] [visor:shutdown:transport_setup.rpc]: Module stopped cleanly. elapsed=659.658_s
[2021-07-14T22:10:39+05:30] INFO [8/10] [visor:shutdown:launcher.proc_manager]: Shutting down module...
[2021-07-14T22:10:39+05:30] INFO (STDERR) [proc:skysocks:551b55409a564389a7e4f92c33ad1c63]: ==================
[2021-07-14T22:10:39+05:30] INFO (STDERR) [proc:skysocks:551b55409a564389a7e4f92c33ad1c63]: WARNING: DATA RACE
[2021-07-14T22:10:39+05:30] INFO (STDERR) [proc:skysocks:551b55409a564389a7e4f92c33ad1c63]: Read at 0x00c0000771a8 by goroutine 11:
[2021-07-14T22:10:39+05:30] INFO (STDERR) [proc:skysocks:551b55409a564389a7e4f92c33ad1c63]:   github.com/skycoin/skywire/internal/skysocks.(*Server).Close()
[2021-07-14T22:10:39+05:30] INFO (STDERR) [proc:skysocks:551b55409a564389a7e4f92c33ad1c63]:       /home/erson/Workspace/Go/skywire/internal/skysocks/server.go:83 +0x64
[2021-07-14T22:10:39+05:30] INFO (STDERR) [proc:skysocks:551b55409a564389a7e4f92c33ad1c63]:   main.main.func1()
[2021-07-14T22:10:39+05:30] INFO (STDERR) [proc:skysocks:551b55409a564389a7e4f92c33ad1c63]:       /home/erson/Workspace/Go/skywire/cmd/apps/skysocks/skysocks.go:59 +0x53
[2021-07-14T22:10:39+05:30] INFO (STDERR) [proc:skysocks:551b55409a564389a7e4f92c33ad1c63]:
[2021-07-14T22:10:39+05:30] INFO (STDERR) [proc:skysocks:551b55409a564389a7e4f92c33ad1c63]: Previous write at 0x00c0000771a8 by main goroutine:
[2021-07-14T22:10:39+05:30] INFO (STDERR) [proc:skysocks:551b55409a564389a7e4f92c33ad1c63]:   github.com/skycoin/skywire/internal/skysocks.(*Server).Serve()
[2021-07-14T22:10:39+05:30] INFO (STDERR) [proc:skysocks:551b55409a564389a7e4f92c33ad1c63]:       /home/erson/Workspace/Go/skywire/internal/skysocks/server.go:39 +0x5b
[2021-07-14T22:10:39+05:30] INFO (STDERR) [proc:skysocks:551b55409a564389a7e4f92c33ad1c63]:   main.main()
[2021-07-14T22:10:39+05:30] INFO (STDERR) [proc:skysocks:551b55409a564389a7e4f92c33ad1c63]:       /home/erson/Workspace/Go/skywire/cmd/apps/skysocks/skysocks.go:65 +0x7a4
[2021-07-14T22:10:39+05:30] INFO (STDERR) [proc:skysocks:551b55409a564389a7e4f92c33ad1c63]:
[2021-07-14T22:10:39+05:30] INFO (STDERR) [proc:skysocks:551b55409a564389a7e4f92c33ad1c63]: Goroutine 11 (running) created at:
[2021-07-14T22:10:39+05:30] INFO (STDERR) [proc:skysocks:551b55409a564389a7e4f92c33ad1c63]:   main.main()
[2021-07-14T22:10:39+05:30] INFO (STDERR) [proc:skysocks:551b55409a564389a7e4f92c33ad1c63]:       /home/erson/Workspace/Go/skywire/cmd/apps/skysocks/skysocks.go:56 +0x779
[2021-07-14T22:10:39+05:30] INFO (STDERR) [proc:skysocks:551b55409a564389a7e4f92c33ad1c63]: ==================

Expected behavior
Close without data race.

Additional context
This was observed in Interactive test environment in skywire-services.

Possible implementation
Add a mutex lock