rpc: make gettxoutsettinfo run lock-free

For leveldb "An iterator operates on a snapshot of the database taken
when the iterator is created". This means that it is unnecessary to
lock out other threads while computing statistics, and neither to hold
cs_main for the whole time. Let the thread run free.

src/rpcblockchain.cpp

@@ -345,8 +345,6 @@ UniValue gettxoutsetinfo(const UniValue& params, bool fHelp)
             + HelpExampleRpc("gettxoutsetinfo", "")
         );
 
-    LOCK(cs_main);
-
     UniValue ret(UniValue::VOBJ);
 
     CCoinsStats stats;