use C++11 thread, each for a query;
lock teh whole database when load/build/unload;
lock teh database when running update query;
query cache and the whole KVstore not resolved;
debug_lock should be removed later;
A query thread can open file in Util::memoryLeft once, and may also use Stream which will also open several files, so the limits of system must be increased to support high parallism;
by zengli, bugs to do