reuse buffered event objects in EventManager to avoid new/delete hammering

This does two things:

* adds a boost::singleton_pool that is used for Event new/delete memory handling (faster than standard allocator)

* modifies the EventManager to reuse a list of allocated events.  This avoids unnecessary construction/destruction and (de)alloc of Events.  This list is allocated once when the event manager is created and dealloced when the event manager dies.

This PR improves cep17 inpro from 1.5 min to 50 sec (on my machine).