Initialize start time for server (#2220)

* Initialize start time for server

By initializing start_time_ for server, I hope to measure the time taken to process a request at the end maybe in the set_logger callback and print it.

I only see current usage in client with server retaining the inital min value

* Add test to verify start time is initialized

* Address review comments

* run clang format

httplib.h

@@ -8264,6 +8264,7 @@ Server::process_request(Stream &strm, const std::string &remote_addr,
   if (!line_reader.getline()) { return false; }
 
   Request req;
+  req.start_time_ = std::chrono::steady_clock::now();
 
   Response res;
   res.version = "HTTP/1.1";