nsqd: fix flush race

[mreiferson]

Upgraded to Go 1.9 and was running benchmarks and observed this panic:

$ go test -v -run None -bench ProtocolV2MultiSub*
goos: darwin
goarch: amd64
pkg: github.com/nsqio/nsq/nsqd
BenchmarkProtocolV2MultiSub1-4    	fatal error: concurrent map iteration and map write

goroutine 39 [running]:
runtime.throw(0x14f434f, 0x26)
	/usr/local/Cellar/go/1.9/libexec/src/runtime/panic.go:605 +0x95 fp=0xc4203c97e0 sp=0xc4203c97c0 pc=0x102d325
runtime.mapiternext(0xc4203c99a8)
	/usr/local/Cellar/go/1.9/libexec/src/runtime/hashmap.go:778 +0x6f1 fp=0xc4203c9878 sp=0xc4203c97e0 pc=0x100ba11
github.com/nsqio/nsq/nsqd.(*Channel).flush(0xc420148000, 0x0, 0x0)
	/Users/mreiferson/dev/src/github.com/nsqio/nsq/nsqd/channel.go:224 +0x354 fp=0xc4203c9a18 sp=0xc4203c9878 pc=0x1385f14
github.com/nsqio/nsq/nsqd.(*Channel).exit(0xc420148000, 0xc4203c9b00, 0x0, 0x0)
	/Users/mreiferson/dev/src/github.com/nsqio/nsq/nsqd/channel.go:176 +0x293 fp=0xc4203c9af0 sp=0xc4203c9a18 pc=0x1385883
github.com/nsqio/nsq/nsqd.(*Channel).Close(0xc420148000, 0xc420079650, 0xc4203c9bc8)
	/Users/mreiferson/dev/src/github.com/nsqio/nsq/nsqd/channel.go:141 +0x30 fp=0xc4203c9b20 sp=0xc4203c9af0 pc=0x13855c0
github.com/nsqio/nsq/nsqd.(*Topic).exit(0xc42008c2c0, 0xc4203c9d00, 0xc4203c9d30, 0xc4203c9d20)
	/Users/mreiferson/dev/src/github.com/nsqio/nsq/nsqd/topic.go:343 +0x32c fp=0xc4203c9c98 sp=0xc4203c9b20 pc=0x13b5a1c
github.com/nsqio/nsq/nsqd.(*Topic).Close(0xc42008c2c0, 0xc420078750, 0xc4203c9d18)
	/Users/mreiferson/dev/src/github.com/nsqio/nsq/nsqd/topic.go:305 +0x30 fp=0xc4203c9cc8 sp=0xc4203c9c98 pc=0x13b56c0
github.com/nsqio/nsq/nsqd.(*NSQD).Exit(0xc4200f8000)
	/Users/mreiferson/dev/src/github.com/nsqio/nsq/nsqd/nsqd.go:477 +0x1c2 fp=0xc4203c9d88 sp=0xc4203c9cc8 pc=0x139e802
github.com/nsqio/nsq/nsqd.benchmarkProtocolV2MultiSub(0xc42015c600, 0x1)
	/Users/mreiferson/dev/src/github.com/nsqio/nsq/nsqd/protocol_v2_test.go:1869 +0x616 fp=0xc4203c9f18 sp=0xc4203c9d88 pc=0x13ea876
github.com/nsqio/nsq/nsqd.BenchmarkProtocolV2MultiSub1(0xc42015c600)
	/Users/mreiferson/dev/src/github.com/nsqio/nsq/nsqd/protocol_v2_test.go:1872 +0x34 fp=0xc4203c9f38 sp=0xc4203c9f18 pc=0x13ea934
testing.(*B).runN(0xc42015c600, 0x2710)
	/usr/local/Cellar/go/1.9/libexec/src/testing/benchmark.go:141 +0xb2 fp=0xc4203c9f78 sp=0xc4203c9f38 pc=0x10eb832
testing.(*B).launch(0xc42015c600)
	/usr/local/Cellar/go/1.9/libexec/src/testing/benchmark.go:291 +0x84 fp=0xc4203c9fd8 sp=0xc4203c9f78 pc=0x10ebfe4
runtime.goexit()
	/usr/local/Cellar/go/1.9/libexec/src/runtime/asm_amd64.s:2337 +0x1 fp=0xc4203c9fe0 sp=0xc4203c9fd8 pc=0x105d101
created by testing.(*B).doBench
	/usr/local/Cellar/go/1.9/libexec/src/testing/benchmark.go:260 +0x70

[mreiferson]

RFR

[jehiah]

👍🏻