[2637] Use logger to issue the warning about using memfile lease backend.

src/lib/dhcpsrv/dhcpsrv_messages.mes

@@ -226,6 +226,12 @@ a database backend, but where no 'type' keyword has been included in
 the access string.  The access string (less any passwords) is included
 in the message.
 
+% DHCPSRV_USING_MEMFILE_BACKEND using memfile database backend, leases will be lost after restart
+This warning message is issued when the server is configured to use default
+'memfile' backend to store leases. The leases held by this backend are
+lost on server's restart. Currently 'memfile' backend is usable for testing
+purposes mainly.
+
 % DHCPSRV_UNKNOWN_DB unknown database type: %1
 The database access string specified a database type (given in the
 message) that is unknown to the software.  This is a configuration error.

src/lib/dhcpsrv/memfile_lease_mgr.cc

@@ -22,8 +22,7 @@ using namespace isc::dhcp;
 
 Memfile_LeaseMgr::Memfile_LeaseMgr(const ParameterMap& parameters)
     : LeaseMgr(parameters) {
-    std::cout << "Warning: Using memfile database backend. It is usable for limited"
-              << " testing only. Leases will be lost after restart." << std::endl;
+    LOG_WARN(dhcpsrv_logger, DHCPSRV_USING_MEMFILE_BACKEND);
 }
 
 Memfile_LeaseMgr::~Memfile_LeaseMgr() {