Cleanup: Simplify MEMPROXY_CLASS_* macros

Cleanup: Simplify MEMPROXY_CLASS_* macros

There are no logical changes in this patch.

* Make MEMPROXY_CLASS() macro inline its definitions.

* Remove MEMPROXY_CLASS_INLINE

* Remove last remaining mention of obsolete MEMPROXY_CLASS_INIT

* Adjust MEMPROXY_CLASS() to control its private/public definitions
  appropriate to their needs.

* Shuffle all MEMPROXY_CLASS() macro uses to the top of the relevant
  classes (in some cases renaming struct to class with public members
  if necessary) such that the new terminal private: does not affect
  existing class definitions.