web_service: follow-up fix to #4842 ...

* The web_service http request is now fixed on Windows (R) platform.
* The issue is due to a complicated race-condition in `httplib`, a detailed
  explanation is available at https://github.com/yhirose/cpp-httplib/pull/701
* A pending Pull Request on `httplib` has been applied to remedy the
  said race-condition.
* The socket availability check is removed due to a behavioral chice of
  `httplib` that a socket will not be created before any actual request
  is sent.

externals/httplib/README.md

@@ -1,4 +1,4 @@
-From https://github.com/yhirose/cpp-httplib/tree/fce8e6fefdab4ad48bc5b25c98e5ebfda4f3cf53
+From https://github.com/yhirose/cpp-httplib/tree/ff5677ad197947177c158fe857caff4f0e242045 with https://github.com/yhirose/cpp-httplib/pull/701
 
 MIT License
 

src/web_service/web_backend.cpp

@@ -71,11 +71,6 @@ struct Client::Impl {
             return {};
         }
 
-        if (!cli->is_socket_open()) {
-            LOG_ERROR(WebService, "Failed to open socket, skipping request!");
-            return {};
-        }
-
         cli->set_connection_timeout(TIMEOUT_SECONDS);
         cli->set_read_timeout(TIMEOUT_SECONDS);
         cli->set_write_timeout(TIMEOUT_SECONDS);