mod_h2 v1.15.0

• Code cleanups and Simplifications:
  • in stream instance and main connection output handling for a
    common strategy in h2/h2c versions of the protocol. Stream instances are
    kept in one place which will make future optimizations in state handling
    easier.
  • Discarding idea of re-using bucket beams and let them live for one request
    only. Removing design/implementation overhead of never used features.
  • Making mutexes nested, removing optional lock code no longer necessary.