* database confirm method in different threads, maximum 5 seconds /each check
* close current db connection when error take place.

Add a thread-quitting cleaning method. when a thread is quitting, its finished signal will be caugth,
so that , db resource class can clean the database connections belongs to this thread one by one.
this method is very important in some situation, such as Qt-concurrent threading, threads will be created, deleted frequently.

* sloved multi-thread db access problem. the databaseresource class has been
improved, for each thread, a thread-owned db connection will be cloned from main
db connection.

const QByteArray & type paras, will cause lift cycle problems. using Value-pass instead.
----
QString, QByteArray supports ref-count and auto-delete , also, these classes support
least-copying, deep-copy will only take place, when the data is about to be changed.
so, in singnals and slots, using QString, QByteArray instead of const QString & and const QByteArray & ,
will not cause performance problem.

Add some method to manage db conn resources.

Start database develop