enable connection reuse in libcurl

The Connection Reuse section of the Everything curl book explains how this
works:

https://everything.curl.dev/libcurl/connectionreuse

> When you are using the easy API, or, more specifically, curl_easy_perform(),
> libcurl will keep the pool associated with the specific easy handle. Then
> reusing the same easy handle will ensure it can reuse its connection.

Before, in the `HttpClient::perform` method, a new curl handle was created
upon every invocation, which meant that there was no opportunity to reuse
connections from the underlying connection pool.

By making the curl handle static, it ensures that the same one remains in use,
thus providing the means to reuse the established connection pool.

Fixes Netflix-Skunkworks#51.

spectator/http_client.cc

@@ -199,7 +199,7 @@ auto HttpClient::perform(const char* method, const std::string& url,
                          int attempt_number) const -> HttpResponse {
   LogEntry entry{registry_, method, url};
 
-  CurlHandle curl;
+  static CurlHandle curl;
   auto total_timeout = config_.connect_timeout + config_.read_timeout;
   curl.set_timeout(total_timeout);
   curl.set_connect_timeout(config_.connect_timeout);