Simplify service and IO thread's class hierarchies

Simplify service and IO thread's class hierarchies

* get rid of useless Generic* and Base* classes that do nothing
* shift what little functionality they provided to other entities
* make a few leaf classes final
* devirtualize a few methods across the hierarchy, either by making them
  local or via CRTP
* replace the virtual Event hierarchy and handlers by a single
  statically known Event type

Change-Id: Id3afef98ff99a5b0eb3966f1cfdf0dcaa52cd909