Reduce SHAMapTreeNode copying during SHAMap unsharing:

In some code paths, we bump the SHAMap sequence number
before we unshare. This forces SHAMapTreeNode to be
copied. By making the ledger immutable we cause the
unsharing to occur earlier, eliminating the copies.

src/ripple/app/main/Application.cpp

@@ -1084,6 +1084,7 @@ void ApplicationImp::startNewLedger ()
         firstLedger->updateHash ();
         firstLedger->setClosed ();
         firstLedger->setAccepted ();
+        firstLedger->setImmutable();
         m_ledgerMaster->pushLedger (firstLedger);
 
         Ledger::pointer secondLedger = std::make_shared<Ledger> (true, std::ref (*firstLedger));