Increase socket buffer size to allow ProcessGroup init up to 12k ranks

Summary:
The c10d socket and gloo listener both set their buffer size to 2048 which causes connection issue at 4k scale. This diff sets the buffer size to `-1` which uses `somaxconn` as the actual buffer size, aiming to enable 24k PG init without crash. The experiment shows the ability to successful creation of 12k ranks without crash.

split the original diff for OSS vs. internal.

Caution: we need the change on both gloo and c10d to enable 12k PG init. Updating only one side may not offer the benefit.

Reviewed By: wconstab, bmaurer

Differential Revision: D48617912

fbshipit-source-id: 3ba40d1b94c113a268ded0ea8f51a03daa1233d3

Author: XilunWu

gloo/transport/tcp/listener.h

@@ -32,7 +32,7 @@ class Listener final : public Handler {
   using connect_callback_t =
       std::function<void(std::shared_ptr<Socket> socket, Error error)>;
 
-  static constexpr int kBacklog = 2048;
+  static constexpr int kBacklog = -1;  // allow somaxconn
 
   Listener(std::shared_ptr<Loop> loop, const attr& attr);