rpc: Fix leveldb iterator leak, and flush before `gettxoutsetinfo`

This fixes an iterator leak resulting in

    bitcoind: db/version_set.cc:789: leveldb::VersionSet::~VersionSet(): Assertion `dummy_versions_.next_ == &dummy_versions_' failed."

exception on shutdown.

Also make sure to flush pcoinsTip before calling GetStats() to make
sure we apply them to the current height.

src/rpcblockchain.cpp

@@ -319,6 +319,7 @@ Value gettxoutsetinfo(const Array& params, bool fHelp)
     Object ret;
 
     CCoinsStats stats;
+    pcoinsTip->Flush();
     if (pcoinsTip->GetStats(stats)) {
         ret.push_back(Pair("height", (int64_t)stats.nHeight));
         ret.push_back(Pair("bestblock", stats.hashBlock.GetHex()));

src/txdb.cpp

@@ -104,7 +104,7 @@ bool CCoinsViewDB::GetStats(CCoinsStats &stats) const {
     /* It seems that there are no "const iterators" for LevelDB.  Since we
        only need read operations on it, use a const-cast to get around
        that restriction.  */
-    leveldb::Iterator *pcursor = const_cast<CLevelDBWrapper*>(&db)->NewIterator();
+    boost::scoped_ptr<leveldb::Iterator> pcursor(const_cast<CLevelDBWrapper*>(&db)->NewIterator());
     pcursor->SeekToFirst();
 
     CHashWriter ss(SER_GETHASH, PROTOCOL_VERSION);