Add 'dropCache' function to remove extraneous nodes from memory.

src/cpp/ripple/Ledger.h

@@ -153,6 +153,13 @@ public:
 	// low level functions
 	SHAMap::ref peekTransactionMap() { return mTransactionMap; }
 	SHAMap::ref peekAccountStateMap() { return mAccountStateMap; }
+	void dropCache()
+	{
+		if (mTransactionMap)
+			mTransactionMap->dropCache();
+		if (mAccountStateMap)
+			mAccountStateMap->dropCache();
+	}
 
 	// ledger sync functions
 	void setAcquiring(void);

src/cpp/ripple/SHAMap.cpp

@@ -851,6 +851,15 @@ bool SHAMap::getPath(const uint256& index, std::vector< std::vector<unsigned cha
 	return true;
 }
 
+void SHAMap::dropCache()
+{ // CAUTION: Changes can be lost
+	boost::recursive_mutex::scoped_lock sl(mLock);
+
+	mTNByID.clear();
+	if (root)
+		mTNByID[*root] = root;
+}
+
 void SHAMap::dump(bool hash)
 {
 #if 0

src/cpp/ripple/SHAMap.h

@@ -382,6 +382,9 @@ public:
 	// Returns a new map that's a snapshot of this one. Force CoW
 	SHAMap::pointer snapShot(bool isMutable);
 
+	// Remove nodes from memory
+	void dropCache();
+
 	// hold the map stable across operations
 	ScopedLock Lock() const { return ScopedLock(mLock); }