Packet out Memory Interface (PMI) update; bug fixes

Release 1.17.0

- [API Change] Packet out Memory Interface (PMI) update:
  - Split PMI pool return into pmi_release and pmi_return
  - PMI callbacks take peer_ctx and is_ipv6 arguments
- [BUGFIX] Fix use-after-free when certificate is updated
- Silence gcc warning in optimized mode by performing useless
  initialization
- cmake: use the standard variable CMAKE_BUILD_TYPE instead of
  DEVEL_MODE