Optimize and clean up SHAMap::iterator:

* Remove dependence on boost::iterator_facade.
* Rename iterator to const_iterator.
* Change value_type from shared_ptr<SHAMapItem const> to SHAMapItem.
* Install a stack-path to the current SHAMapItem in the const_iterator.

src/ripple/app/ledger/impl/OpenLedger.cpp

@@ -207,7 +207,7 @@ debugTostr (SHAMap const& set)
     {
         try
         {
-            SerialIter sit(item->slice());
+            SerialIter sit(item.slice());
             auto const tx = std::make_shared<
                 STTx const>(sit);
             ss << debugTxstr(tx) << ", ";