Shutdown notifications go routines

[hannahhoward]

Goals

Resolve go routine leak for notifications publishers

Implementation

• write a benchmark that simulates the conditions where we are seeing this go routine leak
• add a Startup() method to the publisher and don't start go routine automatically (not essential but generally we try not to start go-routines in constructors
• call Shutdown() on the publisher whenever the module using it shuts down (MessageQueue & PeerResponseSender)

Benchmark results before:

go test -v -bench=BenchmarkRoundtripSuccess/test-repeated-disconnects-20-10000$ --memprofile=profile.out --cpuprofile=cpu.out
goos: darwin
goarch: amd64
pkg: github.com/ipfs/go-graphsync/benchmarks
BenchmarkRoundtripSuccess
BenchmarkRoundtripSuccess/test-repeated-disconnects-20-10000
    benchmark_test.go:130: Number of running go-routines: 866
    benchmark_test.go:130: Number of running go-routines: 2447
    benchmark_test.go:130: Number of running go-routines: 9548
    benchmark_test.go:130: Number of running go-routines: 25369
    benchmark_test.go:130: Number of running go-routines: 44630
BenchmarkRoundtripSuccess/test-repeated-disconnects-20-10000-16         	     231	   5326464 ns/op	 4678319 B/op	   69275 allocs/op
PASS
ok  	github.com/ipfs/go-graphsync/benchmarks	5.007s

Benchmark results afterward (wonder what is causing the remaining go routines to still be running... though also this may be coming from some of the dependencies and not graphsync itself):

go test -v -bench=BenchmarkRoundtripSuccess/test-repeated-disconnects-20-10000$ --memprofile=profile.out --cpuprofile=cpu.out
goos: darwin
goarch: amd64
pkg: github.com/ipfs/go-graphsync/benchmarks
BenchmarkRoundtripSuccess
BenchmarkRoundtripSuccess/test-repeated-disconnects-20-10000
    benchmark_test.go:130: Number of running go-routines: 26
    benchmark_test.go:130: Number of running go-routines: 47
    benchmark_test.go:130: Number of running go-routines: 68
    benchmark_test.go:130: Number of running go-routines: 89
BenchmarkRoundtripSuccess/test-repeated-disconnects-20-10000-16         	     213	   4750892 ns/op	 4646892 B/op	   69192 allocs/op
PASS
ok  	github.com/ipfs/go-graphsync/benchmarks	2.819s

[Stebalien]

I'd consider tracking using https://github.com/uber-go/goleak. If we use that liberally, we should be able to detect all leaks.