[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/cluster_tls_test.go

@@ -103,6 +103,7 @@ func TestClusterTLSInsecure(t *testing.T) {
 		cluster {
 			name: "xyz"
 			listen: "127.0.0.1:-1"
+			pool_size: -1
 			tls {
 			    cert_file: "./configs/certs/server-noip.pem"
 				key_file:  "./configs/certs/server-key-noip.pem"
@@ -122,6 +123,7 @@ func TestClusterTLSInsecure(t *testing.T) {
 		cluster {
 			name: "xyz"
 			listen: "127.0.0.1:-1"
+			pool_size: -1
 			tls {
 			    cert_file: "./configs/certs/server-noip.pem"
 				key_file:  "./configs/certs/server-key-noip.pem"

test/norace_test.go

@@ -46,6 +46,7 @@ func TestNoRaceRouteSendSubs(t *testing.T) {
 			write_deadline: "2s"
 			cluster {
 				port: -1
+				pool_size: -1
 				%s
 			}
 			no_sys_acc: true

test/ocsp_test.go

@@ -1012,6 +1012,7 @@ func TestOCSPCluster(t *testing.T) {
 			host: "127.0.0.1"
 			advertise: 127.0.0.1
 			port: -1
+			pool_size: -1
 
 			tls {
 				cert_file: "configs/certs/ocsp/server-status-request-url-02-cert.pem"
@@ -1045,6 +1046,7 @@ func TestOCSPCluster(t *testing.T) {
 			host: "127.0.0.1"
 			advertise: 127.0.0.1
 			port: -1
+			pool_size: -1
 
 			routes: [ nats://127.0.0.1:%d ]
 			connect_retries: 30
@@ -1111,6 +1113,7 @@ func TestOCSPCluster(t *testing.T) {
 			host: "127.0.0.1"
 			advertise: 127.0.0.1
 			port: -1
+			pool_size: -1
 
 			routes: [ nats://127.0.0.1:%d ]
 			connect_retries: 30
@@ -1202,6 +1205,7 @@ func TestOCSPCluster(t *testing.T) {
 		store_dir: '%s'
 		cluster {
 			port: -1
+			pool_size: -1
 			name: AB
 			host: "127.0.0.1"
 			advertise: 127.0.0.1

test/operator_test.go

@@ -341,6 +341,7 @@ func TestReloadDoesNotWipeAccountsWithOperatorMode(t *testing.T) {
 	cluster {
 		name: "A"
 		listen: 127.0.0.1:-1
+		pool_size: -1
 		authorization {
 			timeout: 2.2
 		} %s
@@ -463,6 +464,7 @@ func TestReloadDoesUpdateAccountsWithMemoryResolver(t *testing.T) {
 	cluster {
 		name: "A"
 		listen: 127.0.0.1:-1
+		pool_size: -1
 		authorization {
 			timeout: 2.2
 		} %s
@@ -554,6 +556,7 @@ func TestReloadFailsWithBadAccountsWithMemoryResolver(t *testing.T) {
 	cluster {
 		name: "A"
 		listen: 127.0.0.1:-1
+		pool_size: -1
 		authorization {
 			timeout: 2.2
 		} %s

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 {

test/tls_test.go

@@ -1921,6 +1921,7 @@ func TestTLSPinnedCertsRoute(t *testing.T) {
 	port: -1
 	cluster {
 		port: -1
+		pool_size: -1
 		tls {
 			ca_file: "configs/certs/ca.pem"
 			cert_file: "configs/certs/server-cert.pem"