Data race in state.go:1135

Hi. I've detected the following data race in memberlist 0.5.0:

```
==================
WARNING: DATA RACE
Write at 0x00c000693728 by goroutine 53:
github.com/hashicorp/memberlist.(*Memberlist).aliveNode()
.../vendor/github.com/hashicorp/memberlist/state.go:1135 +0x2e3b
github.com/hashicorp/memberlist.(*Memberlist).handleAlive()
.../vendor/github.com/hashicorp/memberlist/net.go:738 +0x69a
github.com/hashicorp/memberlist.(*Memberlist).packetHandler()
.../vendor/github.com/hashicorp/memberlist/net.go:513 +0x1b9
github.com/hashicorp/memberlist.newMemberlist.gowrap3()
.../vendor/github.com/hashicorp/memberlist/memberlist.go:235 +0x33

Previous read at 0x00c000693728 by goroutine 146:
github.com/hashicorp/memberlist.(*Node).Address()
.../vendor/github.com/hashicorp/memberlist/state.go:58 +0x6b
...

Goroutine 53 (running) created at:
github.com/hashicorp/memberlist.newMemberlist()
.../vendor/github.com/hashicorp/memberlist/memberlist.go:235 +0x1aea
github.com/hashicorp/memberlist.Create()
.../vendor/github.com/hashicorp/memberlist/memberlist.go:246 +0x2e
...

Goroutine 146 (running) created at:
...
==================
```

The idea is that memberlist `Members()` returns a list of pointers to `Memberlist::nodes` array items. Then the external user's code is allowed to read them without any locking. But at the same time, they may get modified internally in Memberlist, resulting in a data race.

Not sure if it's a critical issue and how hard it would be to stumble on it in production. But it's still a race...

Trace Members() and Node.Address() in state.go, then compare the read at state.go:58 with the write in aliveNode() at state.go:1135. Reproduce the reported access with Go's race detector and confirm that the externally returned node data can be read safely while internal membership handling continues.

Written by the indexing model from the issue text.