Expose enableReuseAddr for consumers to set

Summary:
`AsyncServerSocket.h` sets a default of `true` for enableReuseAddr_ [1], which results in sockets being created with `SO_REUSEADDR`.  We do not want this to be the case, so this diff exposes a mechanism for proxygen consumers to set this to false.  By default I'm keeping it set to `true` for everything, so functionality shouldn't change for existing users.

[1] https://www.internalfb.com/code/fbsource/[114ac0fd701a8746f703eebf3efc3e2fc48569d7]/xplat/folly/io/async/AsyncServerSocket.h?lines=1047

Differential Revision: D90789145

fbshipit-source-id: 9ed25853618aa8626ea608cb3140f21eea0c81fb

Author: Shane Fitzpatrick

proxygen/httpserver/HTTPServer.cpp

@@ -161,6 +161,7 @@ folly::Expected<folly::Unit, std::exception_ptr> HTTPServer::startTcpServer(
         bootstrap_[i].socketConfig.fastOpenQueueSize =
             accConfig->fastOpenQueueSize;
       }
+      bootstrap_[i].socketConfig.enableReuseAddr = accConfig->enableReuseAddr;
       bootstrap_[i].group(accExe, ioExecutor);
       if (accConfig->reusePort) {
         bootstrap_[i].setReusePort(true);

proxygen/httpserver/HTTPServerAcceptor.cpp

@@ -45,6 +45,7 @@ std::shared_ptr<const AcceptorConfiguration> HTTPServerAcceptor::makeConfig(
       ipConfig.allowInsecureConnectionsOnSecureServer;
   conf->enableTCPFastOpen = ipConfig.enableTCPFastOpen;
   conf->fastOpenQueueSize = ipConfig.fastOpenQueueSize;
+  conf->enableReuseAddr = opts.enableReuseAddr;
   if (ipConfig.ticketSeeds) {
     conf->initialTicketSeeds = *ipConfig.ticketSeeds;
   }

proxygen/httpserver/HTTPServerOptions.h

@@ -222,6 +222,12 @@ class HTTPServerOptions {
    */
   bool useZeroCopy{false};
 
+  /**
+   * Allow multiple sockets to bind to the same address:port. Enabled by
+   * default.
+   */
+  bool enableReuseAddr{true};
+
   /**
    *  zerocopy enable function
    */