Log the default location of trusted CA certificates when tlsTrustCertsFilePath is not specified (#429)

### Motivation

Currently when `tlsTrustCertsFilePath` is not specified, default
locations of CA certificates will be used. However, these paths are
determined by the compile option when building OpenSSL and they are
invisible to users. If the OS uses a different path, the TLS connection
will fail with no helpful error message.

Author: BewareMyPower

lib/ClientConnection.cc

@@ -18,6 +18,7 @@
  */
 #include "ClientConnection.h"
 
+#include <openssl/x509.h>
 #include <pulsar/MessageIdBuilder.h>
 
 #include <boost/optional.hpp>
@@ -227,6 +228,7 @@ ClientConnection::ClientConnection(const std::string& logicalAddress, const std:
                 }
             } else {
                 ctx.set_default_verify_paths();
+                LOG_INFO("Use " << X509_get_default_cert_file() << " as default CA path");
             }
         }