Optional mutex on Transaction

Replace previous pull request : https://github.com/kdeforche/wt/pull/46

Add the possibility to use Wt::Dbo::Transaction with a single Wt::Dbo::Session shared within multiple static resources. 

It is retrocompatible : the boolean added to manage the lock is optionnal and set to false by default.

Example : 
{
    // The transaction will be protected by the mutex
    Wt::Dbo::Transaction transaction(m_session, true);
    Wt::Dbo::Query<> queryRes = m_session.query<>("my_query");
    [...]
}