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()));