Streamline StatEntity

• define a (private) struct se to keep all data, so that the StatEntity copy constructor and assignment can use the compiler generated copy constructor and assignment operator of se.
• prefer direct member initialization, make default constructor = default, and implement reset() in terms of assignment of default constructed struct se to m_se (instead of implementing the default constructor in terms of reset()).
• fix deadlock between add() and reset() which is triggered (eventually) iff setnEntriesBeforeReset is called with a non-negative value
• move binary operators out of global namespace, use friend functions inside of class scope instead.
• implement toString as friend instead of member
• prefer static_assert overt boost static_assert
• remove redundant destructor definition