Refactor treatment of Ledger:

All handling of Ledger in shared_ptr is modified to
use a const managed object when the context requires
immutable semantics.

src/ripple/app/misc/SHAMapStoreImp.cpp

@@ -241,11 +241,12 @@ SHAMapStoreImp::makeDatabase (std::string const& name,
 }
 
 void
-SHAMapStoreImp::onLedgerClosed (Ledger::pointer validatedLedger)
+SHAMapStoreImp::onLedgerClosed(
+    std::shared_ptr<Ledger const> const& ledger)
 {
     {
         std::lock_guard <std::mutex> lock (mutex_);
-        newLedger_ = validatedLedger;
+        newLedger_ = ledger;
     }
     cond_.notify_one();
 }