Fix inconsistent synchronization in HttpTunnelServer.ServerThread

Previously, ServerThread.lastHttpRequestTime was written while
synchronized on this.httpConnections but was read without
synchronization. This could lead to a read of the field producing the
wrong value and cause premature connection timeout.

This commit moves the call to checkNotDisconnected into a block that
sychronizes on this.httpConnections, thereby ensuring that
lastHttpRequestTime can be read safely.

Closes gh-4668

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/tunnel/server/HttpTunnelServer.java

@@ -274,8 +274,8 @@ public class HttpTunnelServer {
 		}
 
 		private void closeStaleHttpConnections() throws IOException {
-			checkNotDisconnected();
 			synchronized (this.httpConnections) {
+				checkNotDisconnected();
 				Iterator<HttpConnection> iterator = this.httpConnections.iterator();
 				while (iterator.hasNext()) {
 					HttpConnection httpConnection = iterator.next();