Cache the hashes of nodes known to be full below.

Remove the TNByID entries for those nodes in state trees.
This reduces the memory and I/O neded during ledger fetching and prevents
ledger fetching from crushing other caches.

src/cpp/ripple/Application.cpp

@@ -359,6 +359,7 @@ void Application::sweep()
 	getMasterLedgerAcquire().sweep();
 	mSLECache.sweep();
 	AcceptedLedger::sweep();
+	SHAMap::sweep();
 	mSweepTimer.expires_from_now(boost::posix_time::seconds(theConfig.getSize(siSweepInterval)));
 	mSweepTimer.async_wait(boost::bind(&Application::sweep, this));
 }