Enable TCP_NODELAY for all synchronous sockets.

The widely documented poor interaction between the Nagle algorithm and
TCP's delayed ACK feature, when making short successive writes, leads to
unnecessary delays (around 50ms on Linux). This happens in httpcore
whenever a POST request is made, since the headers and body are sent as
two separate calls.

The TCP_NODELAY can be enabled to disable Nagle's algorithm, and
therefore avoid this delay.

The option is already enabled by default for all asyncio and Trio
sockets. It is also enabled by CPython's http.client module (which
urllib and requests use) and by many high-level HTTP libraries found in
other languages, including libcurl, Java's Netty, Rust's reqwest and
Go's standard library, to name a few.

httpcore/backends/sync.py

@@ -1,3 +1,4 @@
+import errno
 import socket
 import ssl
 import sys
@@ -94,6 +95,14 @@ def connect_tcp(
             sock = socket.create_connection(
                 address, timeout, source_address=source_address
             )
+
+            # Might fail in OSs that don't implement TCP_NODELAY
+            try:
+                sock.setsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)
+            except OSError as e:
+                if e.errno != errno.ENOPROTOOPT:
+                    raise
+
         return SyncStream(sock)
 
     def connect_unix_socket(