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/ledger/LedgerMaster.cpp

@@ -394,8 +394,9 @@ public:
         }
         if (didApply)
         {
-           mCurrentLedger.set (ledger);
-           getApp().getOPs ().pubProposedTransaction (ledger, txn, result);
+            ledger->setImmutable (); // So the next line doesn't have to copy
+            mCurrentLedger.set (ledger);
+            getApp().getOPs ().pubProposedTransaction (ledger, txn, result);
         }
         return result;
     }

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));

src/ripple/shamap/SHAMap.h

@@ -274,6 +274,7 @@ void
 SHAMap::setImmutable ()
 {
     assert (state_ != SHAMapState::Invalid);
+    unshare ();
     state_ = SHAMapState::Immutable;
 }
 

src/ripple/shamap/impl/SHAMap.cpp

@@ -75,7 +75,7 @@ SHAMap::snapShot (bool isMutable) const
     newMap.seq_ = seq_ + 1;
     newMap.root_ = root_;
 
-    if ((state_ != SHAMapState::Immutable) || !isMutable)
+    if ((state_ != SHAMapState::Immutable) || isMutable)
     {
         // If either map may change, they cannot share nodes
         newMap.unshare ();