Move code from RealConnection to RealConnectPlan (#7065)

* Move code from RealConnection to RealConnectPlan

Also promote RealConnectPlan to a top-level type, ConnectPlan.

Each RealConnection is now created only once its ready to be used
to carry exchanges. We do the actual connect work in ConnectPlan.

* Move startHttp2() back to RealConnection

* Update FastFallbackExchangeFinder for split RealConnection

okhttp-testing-support/src/main/kotlin/okhttp3/TestValueFactory.kt

@@ -31,6 +31,7 @@ import okhttp3.internal.concurrent.TaskRunner
 import okhttp3.internal.connection.RealCall
 import okhttp3.internal.connection.RealConnection
 import okhttp3.internal.connection.RealConnectionPool
+import okhttp3.internal.connection.RealRoutePlanner
 import okhttp3.internal.http.RealInterceptorChain
 import okhttp3.internal.http.RecordingProxySelector
 import okhttp3.tls.HandshakeCertificates
@@ -176,6 +177,14 @@ class TestValueFactory : Closeable {
       .build()
   }
 
+  fun newRoutePlanner(
+    client: OkHttpClient,
+    address: Address = newAddress(),
+  ): RealRoutePlanner {
+    val call = RealCall(client, newRequest(address), forWebSocket = false)
+    return RealRoutePlanner(client, address, call, newChain(call))
+  }
+
   override fun close() {
     taskFaker.close()
   }