Ledger close time optimizations (RIPD-998, RIPD-791):

Add a LedgerMaster function to get a ledger's
close time from either its hash or sequence number.
Use this function when adding the 'date' fields to
transaction JSON. This avoids constructing large numbers
of ledgers.

src/ripple/app/ledger/LedgerMaster.h

@@ -137,6 +137,12 @@ public:
     virtual uint256 getLedgerHash(
         std::uint32_t desiredSeq, Ledger::ref knownGoodLedger) = 0;
 
+    virtual boost::optional <uint32_t> getCloseTimeBySeq (
+        LedgerIndex ledgerIndex) = 0;
+
+    virtual boost::optional <uint32_t> getCloseTimeByHash (
+        LedgerHash const& ledgerHash) = 0;
+
     virtual void addHeldTransaction (std::shared_ptr<Transaction> const& trans) = 0;
     virtual void fixMismatch (Ledger::ref ledger) = 0;
 

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

@@ -1377,6 +1377,33 @@ public:
         return mCompleteLedgers.toString ();
     }
 
+    boost::optional <uint32_t>
+    getCloseTimeBySeq (LedgerIndex ledgerIndex) override
+    {
+        uint256 hash = getHashBySeq (ledgerIndex);
+        return hash.isNonZero() ? getCloseTimeByHash (hash) : boost::none;
+    }
+
+    boost::optional <uint32_t>
+    getCloseTimeByHash (LedgerHash const& ledgerHash) override
+    {
+        auto node = app_.getNodeStore().fetch (ledgerHash);
+        if (node &&
+            (node->getData().size() >= 120))
+        {
+            SerialIter it (node->getData().data(), node->getData().size());
+            if (it.get32() == HashPrefix::ledgerMaster)
+            {
+                it.skip (
+                    4+8+32+    // seq drops parentHash
+                    32+32+4);  // txHash acctHash parentClose
+                return it.get32();
+            }
+        }
+
+        return boost::none;
+    }
+
     uint256 getHashBySeq (std::uint32_t index) override
     {
         uint256 hash = mLedgerHistory.getLedgerHash (index);

src/ripple/app/misc/impl/Transaction.cpp

@@ -161,10 +161,10 @@ Json::Value Transaction::getJson (int options, bool binary) const
 
         if (options == 1)
         {
-            auto ledger = mApp.getLedgerMaster ().
-                    getLedgerBySeq (mInLedger);
-            if (ledger)
-                ret[jss::date] = ledger->info().closeTime;
+            auto ct = mApp.getLedgerMaster().
+                getCloseTimeBySeq (mInLedger);
+            if (ct)
+                ret[jss::date] = *ct;
         }
     }