[ADDED] Multiple routes and ability to have per-account routes

New configuration fields:
```
cluster {
   ...
   pool_size: 5
   accounts: ["A", "B"]
}
```

The configuration `pool_size` in the example above means that this
server will create 5 routes to a remote server, assuming that that
server has the same `pool_size` setting.

Accounts (which are not part of the `accounts[]` configuration)
are assigned a specific route in this pool, and this will be the
same route on all servers in the cluster.

Accounts that are defined in the `accounts` field will each have
a dedicated route connection. This will allow suppression of the
account name in some of the route protocols, reducing bytes transmitted
which may increase performance.

Signed-off-by: Ivan Kozlovic <ivan@synadia.com>

test/test.go

@@ -79,6 +79,8 @@ func RunServerCallback(opts *server.Options, callback func(*server.Server)) *ser
 	opts.NoLog = !doLog
 	opts.Trace = doTrace
 	opts.Debug = doDebug
+	// For all tests in the "test" package, we will disable route pooling.
+	opts.Cluster.PoolSize = -1
 
 	s, err := server.NewServer(opts)
 	if err != nil || s == nil {