[3070] Added log message for Rapid Commit.

src/bin/dhcp6/dhcp6_messages.mes

@@ -435,6 +435,12 @@ mandatory client-id option. This is most likely caused by a buggy client
 (or a relay that malformed forwarded message). This request will not be
 processed and a response with error status code will be sent back.
 
+% DHCP6_RAPID_COMMIT %1: Rapid Commit option received, following 2-way exchange
+This debug messgage is issued when the server found Rapid Commit option
+in the client's message and the 2-way exchanges are supported by the
+server for the subnet, which the client is connected to. The argument
+specifies the client and transaction identification information.
+
 % DHCP6_RELEASE_NA address %1 belonging to client duid=%2, iaid=%3 was released properly
 This debug message indicates that an address was released properly. It
 is a normal operation during client shutdown.

src/bin/dhcp6/dhcp6_srv.cc

@@ -2321,6 +2321,8 @@ Dhcpv6Srv::processSolicit(const Pkt6Ptr& solicit) {
     if (ctx.subnet_ && ctx.subnet_->getRapidCommit()) {
         OptionPtr opt_rapid_commit = solicit->getOption(D6O_RAPID_COMMIT);
         if (opt_rapid_commit) {
+            LOG_DEBUG(options_logger, DBG_DHCP6_DETAIL, DHCP6_RAPID_COMMIT)
+                .arg(solicit->getLabel());
             // If Rapid Commit has been sent by the client, change the
             // response type to Reply and include Rapid Commit option.
             response->setType(DHCPV6_REPLY);